When two items are given to a player in creative mode using /loot and one is cleared, multiple times in a tick, they are sometimes lost if the inventory is open while they are being given.

2020-10-16 10-34-06.mp4

Attached is a datapack. If you download and use this datapack in a world, you can reproduce this bug easily. Empty your inventory, go into creative mode, and run the command /function bug:start and as you are receiving items, open your item. Once all the items have been deposited, you will get a message in chat. You will notice that some of the stone is missing from your inventory, either multiple stacks or just small amounts from a stack.

 Bug Example.zip

This has only been replicated in version 1.16.3, but purely because I did not test other versions.

This has been replicated on both multiplayer and singleplayer servers. Vanilla Minecraft was used on both.