In my sing player survival world i made a mob farm with completely dark spawning platforms at an altitude of 234 and 237 and waited for mobs to spawn directly beneath the spawning platforms at a Y level 192. At a render distance of 4 no mobs seemed to spawn. As i raised the render distance incrementally mobs only spawned on render distances above 8. As I tried to reproduce this bug in creative mobs still spawned at a Y level of 70 in a redstone ready superflat. The bug the appeared again when I made dark spawning rooms at y levels above 128.