I don't even know if this is a bug, but are some missing MHF heads,like for exemple skeletal horse,Witch,Horse
if I remember well there were Witch skulls.
if it isn't a bug,i'll just ask you if in 1.9 you can add MHF_Witch, MHF_SkeletalHorse, MHF_Horse etc

to see this do /give @p minecraft:skull 1 3

if it gives you steve's or alex's head,try to do "Sheep" instead of "Witch",it'll give you a sheep's skull