Users reported the problem on the Apple Support Communities, MacRumors Forums and Reddit. Based on the comments, the problem most often affects displays connected to Mac mini via HDMI, not Thunderbolt.
Apple did not say when it could fix the problem. Perhaps a solution will come with macOS Big Sur 11.3. Until the update is released, you can try to fix the problem as follows:
- put your Mac mini to sleep;
- wait two minutes and turn it on;
- Disconnect the display from your Mac mini and then reconnect.
- Adjust display resolution in System Preferences> Displays.
If the problem continues after restarting your Mac mini, you must repeat the steps above.