We suggest you to verify the phone in safe mode for 24 hours to diagnose the overheating issue.
Safe mode to check if added apps are causing problems. This mode only loads basic system software and allows you to disable third-party applications.
1. After turning off the device, turn it on.
2. Once the Samsung logo has started, press and hold the volume down button.
3. When the home screen appears, the safe mode is displayed in the lower left corner of the screen.
How to deactivate safe mode.
1. Press and hold the power button.
2. Tap Restart.
Please provide us the brief description of the software bugs and the apps on your phone so that we can help you easily. We suggest you to attach the screenshots which show the issue exactly for better understanding.
Thank you GG2 and Jayman1973 for sharing your feedback and great to know that you liked your Galaxy S21 Ultra.