Mobs Will NOT spawn in my custom dimension

I'm having a horrible pickle trying to get mobs to spawn in a dimension i've created on MCreator 2020.4.

Try as I might, whatever I do, the mobs I have chosen (1 custom mob, as well as chickens and horses) will NOT spawn in that dimension. I have tried:
 

All 4 spawn types for each of the mobs

Setting the dimension to a grass floor

Enabling 'overworld properties' for the dimension

Enabling/disabling skylight

Enabling/disabling global light

Messing with biome settings

I'm very, very frustrated because this feels like a huge flaw/bug with MCreator. Anybody got any ideas?