When an item is /give'd on the same tick as one of that item was dropped, the item will glitch, with the item from /give either not appearing, appearing in a different item stack, or only appearing when picking up the dropped item.

Steps to reproduce:

Create a scoreboard objective tracking a dropped item, e.g /scoreboard objectives add TEST minecraft.dropped:minecraft.diamond (for dropping a diamond)

Set up 2 chained repeating command blocks with the following commands:

give @a[scores=\{TEST=1}] diamond (give players who dropped a diamond a diamond)

scoreboard players set @a TEST 0 (reset the score)

What should happen is that the player should receive a diamond when dropping one

Instead, the items glitch out, often with the /give'd item disappearing.

Attached is a clip of the glitch in action (ignore the sidebar)