Hi, japeyelnoc, thanks for posting your experience. Well, I think it's obvious from all the posts, here and elsewhere, that this is a relatively common issue and not limited to any particular make and model of phone. Not sure if it's limited to Android 11, I'd have to look back, although this has been reported for several years so it's probably not. So far, there have been no definitive solutions posted. Some Samsung employees have posted and have simply said that the phone should be sent back to Samsung for repair; some people have done that, paid a lot of money and the issue has remained. Not sure if representatives of other phone manufacturers or of Google/Android have posted the same totally useless advice.
For what it's worth. See my 3/31, 4/6 and 4/20 posts above. I'm using a Samsung Galaxy A21 running Android 11, I had the issue and had to reboot about once a day since maybe December or January; I tried the above-noted solution (settings-apps-show system apps-Android System-storage-clear cache; then reboot) on 4/1, since then I've only had to reboot once (after clearing the Android System cache again), so I consider this a fairly durable (although not understood) solution. The problem shows up from time to time if I look at notifications, but disappears on its own after maybe ten to thirty minutes.
I sure wish I understood what's going on here, or had a more definitive solution, but I'm happy now.