Wither skeleton spawing quelled by normal skeletons

Wither skeletons are now much more rare. The spawn rate of the packs is normal, but a lot of these consist of normal skeletons. Previously you'd find a pack that would consist of for example 3 wither skeletons and one skeleton - now it's the other way around if not worse. I can usually find packs of 2, 3 or 4 which are just skeletons.