Mobs spawn no mater how high above you go

Ive searched but didnt find this issue. Mobs dont despawn when you move to far away from them. I was working on my base for 5 or 6 consecutive ingame days and when i walked to my tree farm i was met by a hord of flaming mobs that wandered into unloded chucks. This ruins mobfarms because mobs spawn in caves below you no mater how high you are and dont despawn.