Yes all existing pages built before node flattening was turned on display correctly on publish.
I found this in the logs though:
10.11.2023 08:24:50.601 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/p, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/p]
10.11.2023 08:24:50.601 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/p0, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/p0]
10.11.2023 08:24:50.601 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/table, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/table]
10.11.2023 08:24:50.602 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/table0, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/table0]
10.11.2023 08:24:50.603 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/table1, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/table1]
10.11.2023 08:24:50.604 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/ol, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section/ol]
10.11.2023 08:24:50.608 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section0/p, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section0/p]
10.11.2023 08:24:50.608 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section0/p0, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section0/p0]
10.11.2023 08:24:50.608 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section0/p1, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section0/p1]
10.11.2023 08:24:50.615 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section1/ol, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section1/ol]
10.11.2023 08:24:50.615 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section1/p, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section1/p]
10.11.2023 08:24:50.615 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section1/ol0, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section1/ol0]
10.11.2023 08:24:50.620 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section2/table, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section2/table]
10.11.2023 08:24:50.621 *ERROR* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] org.apache.sling.servlets.get.impl.DefaultGetServlet No renderer for extension html, cannot render resource Paragraph, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section2/ol, type=fmdita/components/flattened, cssClass=default, column=0/0, diffInfo=[null], resource=[JcrNodeResource, type=fmdita/components/flattened, superType=null, path=/content/ariel-portals/docs/en/documentation/engineering-references/all-members/er-57/jcr:content/root/section-wrapper/main-par/contentnode/topic/body/section2/ol]
10.11.2023 08:24:50.667 *INFO* [10.2.38.14 [1699622690449] GET /content/ariel-portals/docs//en/documentation/engineering-references/all-members/er-57.html HTTP/1.1] com.adobe.acs.commons.rewriter.impl.VersionedClientlibsTransformerFactory Refreshing client libraries cache, because /etc.clientlibs/settings/wcm/designs/default/resources could not be found