Some Apple M1 Macs are affected by a Fast User Switching bug
Although this does not look like a hardware-related bug, it looks like only Apple M1-driven devices are affected by it. Multiple user reports reveal that their machines display the following behavior: no matter what the user is doing, the screensaver suddenly kicks in and there is no way to get back in right away.
In one case, the locked out user used Remote Access from his MacBook Pro to get in and log out the user on the affected Mac mini M1. Another user who faced this issue on a MacBook Air had to close and then open the lid. For him, the bug surfaced on two separate user account, both of which had the screensaver disabled.
The solution is to simply disable Fast User Switching, but this renders the login feature useless, and this is obviously not an option. Disabling the screensaver for all users has no positive impact, unfortunately.
In addition to the Mac mini and MacBook Air mentioned above, this bug was also encountered by users of the 13-inch MacBook Pro. In all cases, the operating system involved was macOS Big Sur, including the latest 11.1 update. Apple still has to issue a fix at this time, and those affected by this problem should go ahead and submit their feedback on this page.