I made a custom biome, but no hostile entities were spawning, despite of me putting it into the spawn list. I checked the code and I saw the line "this.addSpawn(EntityClassification.CREATURE, new Biome.SpawnListEntry(EntityType.ZOMBIE, 15, 1, 5));", and just out of curiosity I changed "CREATURE" to "MONSTER", and it worked! This is very annoying, and it took me 1 hour to figure it out.
Also I cannot find the code for "Entity.Mob" when it's in the spawn list, so I don't even know if it does anything.. And if it doesn't do anything it should allow the spawning of hostile entities.
Issue comments
I was just wondering the same with my custom Biome. I'm not seeing the mobs that I added to the list, so nothing shows up there other than some skeletons and creepers that cross over into the biome. Changing the code for each to MONSTER like he said does fix it temporarily. :-)
I have changed this to a feature request.