If you name a mob with a Nametag it won't despawn! Even if you go in spectator mode!
BUT if you summon a named mob with
/summon Giant ~ ~ ~ {CustomName:"StopDespawning"} it will despawn!
Thats all about it!
Please fix it!
Environment
Client: WinXP; Java 7; Vanilla
Server: WIn7; Java 7; Vanilla
Duplicate of MC-31090. Working as intended. The PersistenceRequired tag is used to determine if mobs naturally despawn.