I also have same problem.
I had to send back to Samsung, and they fixed something associated with software, now it restarts once every couple days instead 15-20 times a day. Software issue.
I have the same case, at the moment no solution so far.
I've been using Samsung for a long time, this S21 experience has made me tired of using Samsung. This auto restart problem is a serious problem, because when we need our mobile phone, it suddenly can't be used because it's waiting for a restart. Now how Samsung as a global brand react to solve this problem?
Same here. S21 keeps restarting, but did not do it at repair, so they refuse to replace it. no comment..
I've been using Samsung Galaxy for a long time and this time I'm disappointing with this brand.
I think there is an inherent problem in the machine but unfortunately Samsung does not quick to react to make a replacement.
This worked for me: Go to Settings, Apps, and scroll down to "Android System WebView" and tap. Now tap on the 3 vertical dots on the top right and tap "Uninstall updates". Repeat the same for "Google Play services". Now restart your phone and see it solves the problem. If it does, try going to Settings & Apps and update as required. Mine has worked fine after updating these two apps.
How were you able to get Samsung to issue you a replacement phone before returning the defective phone? I am having the exact same issue with my S21 Plus restarting as you did. Samsung is insisting that I send my phone to Texas for a second evaluation (an authorized Samsung Repair Center already evaluated it and deemed the phone unrepairable). This issue of random, frequent restarts is a known problem with the Samsung S21 series according to the technician at the authorized Samsung Repair Center and numerous comments on these forums for the S21 on Samsung Community and elsewhere. It is clear that the ONLY solution is to either replace the motherboard or get a replacement phone. While the hardware problem is annoying, what is unacceptable is how Samsung makes it unduly burdensome to get a replacement phone by insisting I send it in -- and thus leaving me without a phone for a week (or more). I would like the same solution they offered you, i.e. to get the replacement phone first, make the transfer, and only then return the defective device. This seems a reasonable expectation for a known problem on a premium phone that commands a premium price of $1,000 and is the level of customer service that is commensurate with its competitors, such as Apple which would have simply replaced my phone at the authorized repair center.
I also got a new S21 replacement from Samsung Indonesia.
I suspect the autorestart problem is a hardware problem, so repairing it may not necessarily solve the problem.