Unlike other mobs, Turtles, Frogs, and Tadpoles don't use the PersistenceRequired NBT tag to determine whether or not they're able to despawn. Instead, they simply cannot despawn at all.
To replicate, summon one of these entities with a PersistenceRequired tag set to 0b, or use /data modify on an existing one to set the tag. Then move several hundred blocks away and return. The entity will still be present.
No animal mobs despawn randomly, that's by design.