All MHF head's model get lost that they are unseen, the command of the head is:

/give @s player_head{SkullOwner:"MHF_Pig"}

You can change the "Pig" to most mobs in this command, I only tested some mobs' head, all of them are unseen, so I think all MHF head are unseen

Most players' head are normal, but one head is unseen too like MHF head(I only find one, but I think it's must more than one head are unseen)

My English is not good, please forgive me