seems to be related to StackReference mocking, did anything change in 3.0 for that?

If you look at the migration doc there are some changes to the mocks: https://www.pulumi.com/docs/get-started/install/migrating-3.0/#changes-to-pulumiruntimemocks (make sure you select python as a language). Have you made those changes?

yeah, I applied those changes, but when you call pulumi.StackReference, it seems they are picking up the fact and a mock is not created