I was making a modpack for my friends and I to enjoy, where at some point I would get the issue of hostile (neutral or passive mobs spawned without any problems) mobs refusing to spawn. It was however able to spawn in mod generated structures which I would presume to have fixed spawns like this modpack, the castle the little brick camps, and any other structure for other modpacks. I narrowed it down to this one when I turned all modpack off and let one be on to test for issues, this seems to be an issue for this mod, other procedurally generated mods like Orcz, Born in Chaos, and any modded or natural spawners are fine in this regard.