Should actor isolation inferred from the overrides declaration be recorded in the attributes?

To check my understanding - if attributes were not added, then files importing modules would take more time to compile, but result still would be correct, right?

My main question is why attributes are not synthesised when they are inferred from overriden declarations? In the original implementation it was working, but in the current implementation, it doesn't anymore. Is this an intentional change, or a bug?