When a Trial Spawner becomes ominous, any existing mobs it spawned will despawn. However, if those existing mobs had CanPickUpLoot:1b, then they can despawn with the player's picked up items.

To reproduce:

1. Activate a non-ominous Trial Spawner that generated within a Trial Chamber
2. Find a mob with CanPickUpLoot:1b, this may take a while
3. Give the mob armor or a weapon
4. Drink an ominous bottle to make the Trial Spawner become ominous
5. Watch as the existing mob deletes the items you gave it