The Samsung Galaxy S21, S21+ and S21 Ultra units in the US received the stable version of One UI 4 last week, even the carrier phones. However, one bug managed to sneak through the beta tests and is currently affecting users.
The issue is with the 120 Hz mode, specifically that the screen drops down to 60 Hz as soon as the user lifts their finger rather than, say, when the scrolling animation is done.
Samsung is aware of the issue and is working on a fix. It seems that this issue was actually caught in the beta tests, but the fix that is present in the current version of the software (BUK7) only works on some units. A more complete fix should arrive with the next update.
Initial reports indicated that this issue affected only US users, but Galaxy S21 owners from other regions have also complained of the same problem. Have you noticed any issues on your S21 after the update?