What I did:

I gave a piglin a turtle helmet

What I expected to happen:

When piglin wears turtle helmet, it is displayed above his head

What actually happened:

Its whole head went black/purple due to missing texture, BUT snout and ears were still sticking out, probably because those are external entity models which are not affected by the helmet texture

Why I think this is happening:

Because Mojang forgot about piglins wearing turtle helmets, and therefore gave them the ability to pick em up (head armor item, or smth, can't remember the exact java notation) and then forgot to give a texture for it.

Steps to reproduce:

1) Run command:

/summon piglin~ ~ ~ {ArmorItems:[{},{},{},\{id:"minecraft:turtle_helmet",C:ount:1b}]}

2) Piglin's head texture is purple, as it has no valid file for that texture