If you have your inventory completely filled in creative mode and pick up an item that cannot fit into your inventory, the game picks it up anyway but instead of keeping the item, it is deleted.
Linked issues
Comments

If you have your inventory completely filled in creative mode and pick up an item that cannot fit into your inventory, the game picks it up anyway but instead of keeping the item, it is deleted.
migrated
(Unassigned)
Unconfirmed
(Unassigned)
inventory, item
1.15.2, 20w15a
Retrieved
That has been in the game for years, it's not a bug, that's how it's supposed to work.