The bug

When a dolphin picks up an item, it gets permanently marked as persistent, as with all mobs that can pick up items. However, dolphins pick up their items only for a brief moment so they can play with them. If a dolphin ever touches an item, it cannot despawn in the future.

This makes it impossible to distinguish between dolphins that can and cannot despawn (unlike other mobs who usually keep their items permanently as an indication). It also can create a host of random persistent dolphins who, without interaction, will never despawn and thus imbalance the ocean mob spawns.

Preventing dolphins from getting marked as persistent would cause problems because they could despawn while holding items, but removing the persistence marker when they release the item would also cause problems because a named dolphin would be able to despawn after playing with an item.

How to reproduce

1. Spawn a dolphin
2. Throw an item at it for it to play with
3. /data get entity @e[type=dolphin,sort=nearest,limit=1] PersistenceRequired
   → It's set to 1b