in case you didn't find the answer yet, here it is:

The fun fact explains it, you are using different Base objects. When they are in the same file, you are registering with same Base object. When they are in different files, you need to create a Base object in that file, register the model with that object, then import the same Base object to the other file and register it with the same object. Please be careful to avoid circular imports. Hopefully that would work.