I am installing the 14.5 beta2 update last night (however this has happened on non beta releases too) and I wake up with his morning to see the login screen. I enter my password and the update screen is “paused” with 6 minutes to go and then immediately begins completing. I’ve noticed this happening in the past as well and disregarded it. This can’t be normal during an update for the login screen to appear and stop installations.
I was forced to post this in the beta forum by Apple..
"We removed your post “Screen locking while updating macOS - stopping update because it contained information about beta software. "