I was trying to make a mob farm inside a Hyperbox with some dreadful dirt from Mob Grinding Utils. After placing the dirt you can see the particle effects but nothing will spawn. I moved the hyperbox to a different biome with the same effects. Spawners will work just fine. EDIT: I tested with delightful dirt as well and with a patch of grass.

Intended, the hyperbox biome doesn't have any mob spawns in it