I have a dual monitor M1-Macmini system running Big Sur(ly).
It drives me crazy that the screens jump from monitor to monitor overnight and sometimes after rebooting.
Along with that, the taskbar changes from one monitor to the other during a reboot as well.
Do others have this problem with Big Sur?
Nope, things stay in place on my dual screen setup with my M1 mini.
(Pssst, on Mac, it’s called the dock.)
You might look at the “Displays have separate Spaces” preference (in the Mission Control prefs panel, I think).
Pssst, on Mac, it’s called the dock.
Are you saying that it shows that I use Windows for work? 
Yeah, happens here too. Leaving the mouse where the hidden dock should be on either monitor tends to shift it there. That’s how I recover it/move it where I want it. Fine for 80% of the time, but the other 20% it just seems to pick which monitor it will appear on at random.
Where the dock is also links to Option Tabbing through open Apps. That’s the most annoying element of it. When it moves the open apps list to my second monitor for no reason…
I use separate spaces on two monitors, non-m1 but same issue.