As mentioned, the uppermost right corner inventory slot gets item stuck in it only when in the inventory UI though. Items can't be taken out, swapped/replaced with other items and items can't be placed into the slot. However in the chest UI, furnace UI or crafting UI you can move items in and out of that slot. The mouse cursor also gets stuck when trying to move it over that slot and can't be moved through the square but gets stuck on the border.
Linked issues
Comments 3
MC-143776 does, however, this is still an issue in the latest snapshots for 1.16. Also, it only affects one slot for me whereas the other post seems to affect multiple inventory slots
20w20b is listed in that report so probably a duplicate.
Does MC-143776 describe your issue?