Foxes "delete" mob heads upon picking them up

When a Fox picks up a mob head, the mob head is apparently destroyed. The item disappears off the ground, but it can't be seen in the fox's mouth. Once picking it up/destroying it, the fox won't pick up any other non-food items. Upon picking up a food item, the mob head isn't re-dropped. It's just gone. This happens with every type of mob skull, including the dragon head.