@PancakeIdentity it would cause an influx of either skeletons spawning because there wouldn't be any other mobs. The mob cap would consist purely of wither skeletons rather than a mix of mobs, so there would be more wither skeletons spawning
I'm not sure this is a bug. If this were to be used, it would simply make Wither Skeleton farms extremely overpowered and cause wither skeleton skulls to be coming out like they are nothing.
@PancakeIdentity it would cause an influx of either skeletons spawning because there wouldn't be any other mobs. The mob cap would consist purely of wither skeletons rather than a mix of mobs, so there would be more wither skeletons spawning