Hostile Mobs despawn:
When summoning a hostile mob (a bat, a cat and maybe other Mobs too) via a Commandblock, the mob will despawn (even if it has a Name) after going away a certain distance from it and then returning there. It occures only when it has NoAI.
Duplicate of MC-31090. Naming a mob does not prevent it from despawning, when summoning it. You have to set the PersistenceRequired tag to do this.