The Galaxy S21 series of smartphones are the first few Android devices to update to One UI 4, which is based on Android 12. Despite the shiny nature of this software upgrade, an increasing number of users have reported that their high-end displays have become much less stable after the OTA. This is due to a specific bug that will, according to Samsung, be addressed soon.
https://www.notebookcheck.net/Samsung-assures-Galaxy-S21-users-of-a-fix-for-the-phones-new-Android-12-display-issue.580873.0.html