custom name not same as name tag

custom name not same as name tag

What I expected to happen was that a summoned aggressive mob with a custom name will never despawn
I am trying to spawn permanent monsters with out naming them and the custom name data does not act like the name tag by keeping the monster from despawning

*What actually happened was the custom named skeleton despawned after I move far enough away for the map to unrender and the monster was despawned when I went back

1.summon a agresive mob with a custom name
2.move far enough away from it to were the map disipears
3.go back to the monster
4.see that it is gone