So I’m playing on the newest snapshot and I’ve made a mob spawnner higher than y150 in the ocean and mobs aren’t spawning, but they are spawning on the sea level where I made a small platform. Anyone know why?
This seems to be a bug in 18w43c and possibly some earlier versions: https://bugs.mojang.com/browse/MC-138337
Mobs don't spawn above the highest block that was naturally generated with the world. I don't know why this happens, this is a really weird bug. Some people started analysing the code a bit, but it'll likely take a while until it's properly figured out.
For now I would recommend to stay on 1.12.2, it's the best performing and least buggy version. For more details and reasons, see this answer.