It is very common to Samsung smartphones. Go to the service centre and have it repaired. Remember, since S21 Ultra is out of warranty so you'll be charged for a repair fee.
It is likely to happen, but some reasons suggest that it was caused by if you are charging your phone when an update happens. As you update, it causes the phone to heat. So, how about more if your phone is plugged in? So avoid, updating updates while plugged in as the overheating causes some display issues and damage.
