Someone might have already reported this, but I couldn't find anything similar.

For whatever reason when you use the Buffet world generation option, then select Floating Islands, and use dark_forest_hills as your biome no passive mobs spawn in the world. Hostile mobs spawn just fine, but nothing else (except bats for some reason).

I tried Floating Islands, but with other biomes  and had no issues.

If someone could look into this I'd be grateful.

Thanks!