I have noticed inconsistencies with mob spawning at certain Y-values (higher altitudes).
I will explain how I noticed it - I started to build a simple dark room mob trap - the "collection area" was at y-190 above an ocean biome - I then stacked up 23 blocks and built a simple mob trap.
Once the spawning pads were done and it was fully dark I climbed back down...
What did I expect to happen? The mob spawner to be dropping mobs like mad - the only spawnable space close to me was in that constructed mob trap.
What actually happened: Nothing - no mobs spawned. I relogged several times - checked the spawning pads for light but no mobs would spawn.
I know that mobs become idle at a certain radius but I built it well within that radius.
I also know that no mobs spawn at 240 and above but again the spawning pads were not built anywhere close to that.
Also the spawning pads were 24 blocks away from me - It wasn't a case of me being too close