I also copied the node to the core page dialog to prevent, that any JS or other code of my page obstruct the rendering of the multifield. But there it isn't rendered, too. The render.jsp of the multifield has never been called. There are no errors in the JS console.
And also when I use wcm/foundation/components/page as resource supertype, it doesn't have an effect.
My system properties: Adobe Experience Manager 22.214.171.124