Mob spawning rates depend on how many players are loading the dimension

I built a creeper farm above y200 and have gotten 0 spawns over several hours... The same exact farm in a single player world and in a Minecraft realm world works fine, whereas on a public server with ~5 or so people, it does not work.

The farm's footprint (21x21) intersects 4 chunks, and is over the ocean, at a transitional zone between cold and warm water.

Could it be a bug due to the height, or the biome transition?