MacRumors reports that some M1 Mac models — the 2020 MacBook Air, 2020 13-inch MacBook Pro, and 2020 Mac mini — are susceptible to a Fast User Switching bug that spontaneously activates the screensaver and leaves the user unable to dismiss it.
In macOS Big Sur, Fast User Switching allows users to quickly switch between user accounts without having to completely log out. If you have an M1 Mac and are experiencing the difficulty described above, you can disable Fast User Switching by:
° On your Mac, choose Apple menu > System Preferences, click Users & Groups, then click Login Options.
° If the lock at the bottom left is locked, click it to unlock the preference pane.
° If “Show fast user switching menu as” has a checkmark next to its box, click the box to disable it.
° Click the lock icon to save the change.
Once Apple has fixed the bug, you can repeat these steps, then click the box next to “Show fast user switching menu” to re-enable the feature.