Due to the changes made earlier in I believe... 13w38a. There were changes on how mobs would spawn in chunks and this causes problems with them spawning above 79. I don't know if this issues has been reported already and or is planned to be fixed or is for a future feature, I just thought it might have been a bug in the mob spawning.

Natural mob-spawns ( above LC 79 ( To my knowledge )

NOT affected;
Mob Spawner's