Hi @mark.breuss,
Thanks for this report! We have tracked down the problem with Anvil that was causing this issue, and fixed it. The fix will go live sometime in the next 24 hours.
For reference, you should be able to pass a fully-qualified package path to render_form(), eg render_form("My_Dep_Package.Form1"), and that should work from both the main app and the dependency. (This functionality had a bug which is now fixed!)