I have this issue as well, but with a permanently connected Dell 4K monitor and waking from display sleep. The second low res monitor is fine. Every time I log in, the Dell windows move to the laptop display. This was a problem I solved in Catalina simply by waiting until the monitor powered up from display sleep. Now with Big Sur, nothing I can do will stop it. It is a real pain, as the whole point of the Dell monitor (used vertically) is that I can have multiple large windows. (Macbook Pro 16" 2019)

Hrmm - hard to reply as Firefox 81.0.1 (64-bit) under Catalina doesn't give a reply box! That could be another thread (8-)Safari is fine (no idea why preview shows this line huge and bold. Maybe my ASCII smiley) Simple - start up in recovery mode (Command R) and restore from Time Machine backup. The OS is shown, and mine were *all* Catalina. Seems TM uses the MacOS it was created on. I have just done that. Took ages but was flawless. Many reasons: 1) My VMware Fusion 11 (just bought) doesn't work. I would have to buy V12. 2) Windows on my Dell monitor move onto main LED screen on wake from sleep. 3) Network connections keep dropping. 4) Noticeably slower running