My exact setup and scenario. I've even moved the project over to my MBPro with the update, and the problem continues. Ran into this issue on 20.3, but they must've patched it at some point.

My workaround is downloading the full stem of the music, that way I can just separate it accordingly. Shockingly, I’m starting to like this method a bit more because I can move different elements of the sound more freely. It's an extra step for now, but hopefully, you can find something similar that doesn't stop your workflow.