If a drowned, squid, or a dolphin is named, it still despawns, unlike other mobs.

How to reproduce:
Method 1: Spawn Egg
1. Rename a dolphin, squid, or drowned spawn egg.
2. Use the spawn egg (in water).
3. Go far away and wait.
4. Come back.
5. The mob should be despawned.
Method 2: Name Tag
1. Find a dolphin, squid, or drowned.
2. Rename a name tag.
3. Use the name tag on the mob.
4. Go far away and wait.
5. Come back.
6. The mob should be despawned.