I tried to recreate this on my site also running 2.9.0.beta7, but was unable to reproduce this behavior.
What browser were you viewing your site with? It’s interesting that the content was present when you inspected the page, but was not actually displaying anything. For my testing I was using Chrome Version 103.0.5060.134 and Firefox 91.12.0esr.
Do you recall generally what kind of content was in the Doc topic you make into a published page? I’m wondering if there might be content from another plugin in the post that could be causing this issue besides the Doc plugin, or if the content was just using core Discourse features (plus the Docs plugin).
If you create a topic with the exact same content as the one that’s showing the blank published page, but never include the tags to make it into a Doc, is the issue still present? It’s odd that the issue with the publish page topic persisted after removing the Docs tag.