If a ContentTypeDescriptor is added as a ContentTypes relationship, and then that topic is saved, then the PermittedContentTypes count should be reset to reflect the change. This is evaluated by the Save_ContentTypeDescriptor_UpdatesPermittedContentTypes unit test, which was passing, but that appears to be a false negative. If the content types are first assigned identities, the test no longer passes. It's not immediately clear why the topics being previously saved would impact this, but it calls into question whether this was ever working as expected.