I assume you're using Core Components Page component. It looks like
you're running into:
https://github.com/adobe/aem-core-wcm-components/blob/master/bundles/core/src/main/java/com/adobe/cq/wcm/core/components/internal/Utils.java#L94...based
on the above code, you could add a vanity URL to the target page that is
the page URLe with minus the ".html" and it looks like that should work.
If you wanted to write come code, you could overlay the Core Components
Page's "redirect.html" script, and provi...