aura,
Maybe I didn't include enough detail in my post. I have a 2019 Mac Pro which came with a 580X card.
Somewhere along about May I bought the 5700X MPX from Apple to replace the 580X. At first it caused some problems with VMWare Fusion + Windows 10. I forget which Big Sur version that was (maybe my post above says?), but after the update to Big Sur version 11.4, no more problems. It now has 11.5 and continues to be OK -> no more flaky behavior.
I don't have much non-Apple hardware; some SSD cards and a couple of hard drives. Ditto software; some video editing software and maybe a couple of other things.
Hope this helps,
Tom
EDIT: The problems were associated with Big Sur 11.3.1