When a resource pack modifies the model of a skull/head, but not the base mob's texture (eg by creating a custom model data variant), the item displays correctly, but the worn model is that of the mob directly, still.

Download the attached resource pack, and give yourself a skeleton skull with CustomModelData:1
You'll see the item model changed, but when worn still shows a skeleton skull.

That it uses the entity directly for non-dragon non-piglin heads seems to be a leftover from when the item forms were 2D textures. The dragon and piglin heads are the only ones that actually need it, as the mouth opens and closes and ears wiggle when moving, but perhaps this animation could be specialcased to only apply when it uses the buildin/entity model parent, and no elements.