Mobs not spawning

I'm on 1.16 trying to use the new mob spawning mechanic to build a mob grinder. I'm at the right distance for them to spawn with a massive roof to darken the area. My standing platform is at 175 but nothing spawns. I go down to ground level after a while and nothing else spawns, passive and hostile alike, even at night. It's like being high up breaks the spawning mechanic