I'm experiencing it too on my S20 5G. It feels like it most often happens when it's charging (wirelessly) and reaching 100%, though not always, as in: it's charged a bunch of times without problems but when I do notice it happening/happpened the battery shows 100% when I look at it. Not sure if I've seen it happpen while charging using a cable but I don't do that often. Besides that, it's rebooted in my pocket (locked, not charging) at least twice over the last week or so, so it's not necessarily tied to charging for me. I noticed those times because my Watch3 buzzed that it lost connection.
edit: wanted to reply to AndrewL but it seems that went wrong.
@AndrewLit's happening multiple times a day now, always when it's charging, not necessarily exactly when hitting 100% charge but it could be a bit after that too. Is this being looked at at all?
Edit: december update landed, no change. Still rebooted. I changed the setting for battery life protection so it stops charging at 85%, and it rebooted mere minutes after hitting 85% now. I made a routine now to sound an alarm at 95% battery so I can take it off the charger but it must be obvious this is annoying as hell.