Same here. If I unplug an external display and then plug it back in, none of my apps' windows (which were on the external display’s Space) are visible. The external monitor’s Space appears completely empty. The steps to reproduce the bug are simple and likely affecting a ton of users: Plug in your external display, and then put some app windows in that Space (like Safari and Terminal, or even Chrome). Unplug your display. Plug your display back in. Those windows that you had opened in that Space are no longer visible. This is a showstopper bug that exists in the latest Big Sur public beta (as of Nov. 3). I recommend everyone who sees this (and has this bug) to file a report through the Feedback Assistant app.

I'm also experiencing this bug. If you unplug an external display and then plug it back in, none of your apps’ windows (which were on the external display’s Space beforehand) are visible or where they used to be. Usually for me, the external monitor’s Space appears completely empty. There's a Reddit thread about it here reddit.com/r/apple/comments/ka3wz9/videodemonstrationofhowbigsurforgets_where

Good luck guys, and please fix this Apple!