Does this happen with other messaging apps too, like Google Messages, or is only occuring with FB Messenger?
If this only happens with FB Messenger, the app is most likely the source of the problem. Kindly update here when you find out.
As a low level computer nerd, I can't understand what code would be written in FB to have it give up the reigns of being the default app sheet every reboot of the phone. It's the phone's settings that are changing without my permission. Ie. I'm not in control of the settings.
I have had FB Messenger for many years on all my devices and have never had a single problem like this. I currently have an S22 Ultra.
It shouldn't make a bit of difference to be honest, plus I haven't heard of many other S21 users with this issue. Any news on if this occurs with other apps too yet? We can move forward with some troubleshooting steps if this is happening across the board with many different messaging apps.
Did you already test Google Messages, or a different messaging app? I wouldn't know what would make the app behave like this, but I think it's very possible that it is the culprit. If it's not, there are a couple things we can try out.