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 recently am having random rebooting on my S21 Ultra with Tmobile. My phone worked flawlessly until the first system update about 6 months ago.
I then could not use my 1x camera; it would always remain out of focus, even in pro mode. EDIT: this feature seems to be working again; it's unfortunate the lens was not working while I was on vacation.
Then I started having issues with internet connectivity. Few times a day, wifi or 5g/4g will show zero service, cut all internet, and reconnect in about 10 seconds only to repeat in 30 seconds to a minute later. This will happen a dozen or so times before correcting itself.
The person talking on the phone does not always hear me. Does not matter if I'm on bluetooth, speakerphone, or headset. Simply hanging up and calling again a time or two will fix it.
After recently having issues now with rebooting, I had been hesitant to install the latest update. But I finally caved and updated to the latest firmware hoping this all would be resolved. It has not, however, fixed the rebooting and connectivity issues I have been experiencing and reboots more frequently now.
With all of the others having the same issue with system rebooting, I do not understand why Samsung, with one of their premium flagship phones, does not recognize this problem and issue a voluntarily recall.
It makes one wonder if this is a tactic used to get the customer to spend more money on another phone. Either way, this to me, sounds like a class action lawsuit waiting to happen.