Exact same system as you, Belkin Thunderbolt 3 Pro dock here. Same symptoms on 11.4. I've had the logic board in this machine replaced twice now, bugs logged with Apple support etc. It's very easy to reproduce as others say. This, in addition to the ethernet driver not working as of Big Sur is another reason I think Apple's QC is going down the pan. Totally unacceptable to have this critical bug in a release OS.