The only versions I tried this bug in are 1.16, 1.16.1, 1.15.1, 1.15.2, 1.15, 1.14(1,2,3,4) and all of them had the bug. so basically if you're in any creative mode menu except the inventory one and you are throwing items on the ground while your hot bar is full the items do not go into your inventory, they totally disappear. Then when you do the same thing in the inventory tab after the first scenario the item goes into your inventory, but not in the slot it's supposed to, it mises one slot. So that points to the fact that the item is/was there. but when I inspect more closely there is NOT ANY item in the skipped slot.

BIG BUG