The bug
Having a full inventory and trying to pick up an item while holding F causes the item to visually disappear. Reopening the inventory and clicking on the item causes it to reappear, however the item can be permanently deleted if you log off at the right time.
How to reproduce
Have a full inventory (Including off-hand slot) and one item on the ground
Drop one item
Hold the F key down over the now empty slot
Repeat steps 2-3 until the item disappears
Example
Here is an example in video:
[media]Linked issues
Attachments
Comments

Confirmed in 1.16 Release Candidate 1
Probably related to MC-199643
This can also happen if you shift click an item from a crafting table output and have a repeating command block clearing that item