this happens every once in awhile where the hot bar items glitch out with static. it hadn't happened since 1.16.3 or 4 and it will clear most times when the game is shutdown, left for a few then restarted. Stacked items do retain the number and I will attach a screenshot. I believe it happens while crossing chunk boarders and sticks till a full restart. In previous versions of the issue you'd get a hold on sounds but not this time. I also believe this is the second time it effected the held item.