It seems that some Apple M1 Mac users are experiencing problems with the fast user switching bug. The bug is said to activate the screensaver, not allowing the user to dismiss it. If you’re unaware, in MacOS Big Sur, the fast user switching feature lets you move between user accounts quickly, without spending extra seconds to log out. However, based on some posts by users on Macrumors forums, Apple’s community forums and Reddit, it appears that the screensaver can sometimes handle the entire screen of an Apple M1 MacBook Pro-based Apple M1 MacBook Pro device, leaving the workspace completely unavailable is.
Apple M1 MacBook Pro, Apple M1 MacBook Air, Apple Mac Mini
Some people also reported that the screensaver appears on the screen with a moving mouse pointer but that does not work either.
Users have to close and reopen the lid on their Apple M1 MacBook Air and MacBook Pro devices or press the power button lightly, or use the Ultra-Command-Q key combination to access the login page and continue working.

The only temporary solution is to shut down now or disable the fast user switching feature. However, this leaves the login feature inaccessible. Some have even tried to disable the screensaver in system preferences for all users, but it doesn’t even matter.
This problem has been cropped in the MacBook Air, Apple M1 MacBook Air, 13-inch MacBook Pro and Apple Mac Mini trio. It is also said that macOS will occur on all versions of Big Sur, including the new 11.1 update.
However, given Apple’s history of quickly fixing the issue, users can expect a maintenance update in the coming days.