Very recently I have been having an issue with my hotbar, which only seems to be happening in flat worlds. There are 2 main things it does:

Firstly, it gives me a very hard time trying to switch items. If I select a different slot, the outline will show on that item in the hotbar, but what is actually in my hand and what I’m using has not changed. If I want to switch I will have to tap the slot I want to switch to many times.

Secondly, when I try to switch items in the hotbar, the outline will sometimes spammingly switch between the original slot and the slot I’m trying to switch to until it burns out like a redstone torch. In the end, I am still selected on the original slot.

That is all it does. Once again I’m fairly certain it only does this on flat worlds and that it came with the 1.16.20 update.