It seems to me that you forgot to use the
AssemblyDiscoverable attribute in the project where you have the generated classes for your page types. Please see the docu page here. There is the Allowing discovery of the generated classes in custom assemblies section which describes the details. For the .NET core project, the AssemblyInfo is auto-generated, so you can either turn it off and handle it by yourself or modify the project file and add this:
We use this custom attribute to optimize the discovery of custom implementations.
Hopefully, this helps.