Bug only present in 1.17.1. 1.17 and below did not appear to have this issue.

When in creative mode and the player's inventory is full, dropped items will appear to be picked up by the player, but are instead destroyed.

Steps to reproduce:
1. Fill your inventory (hotbar and inventory) with any item.
2. Use a dispenser, another player, or another method to drop an item
3. Attempt to pick the item up.