Dolphins disappearing even after naming them

Put the summary of the bug you're having here

What I expected to happen was...: 

I expected the dolphin to never despawn once I put a name tag on it. 

What actually happened was...:

It despawned, I don't know why. Maybe from walking too far away or opening and closing realms.

Steps to Reproduce:
1. Spawn a dolphin.
2. Using a name tag, name it.
3. Go far away or open and close realms.