Summary:
If the fox is given an item that can be placed in the armor slots and fox picks it up, then the resulting item will not be in the fox's mouth, but in the armor slots, what causes items in the mouth to not be visible

Steps to reproduce:

1. Summon the fox.
2. Throw the fox an equippable item.
3. Give the fox an item (armor, mob head, etc.) via the /replaceitem command (for example /replaceitem entity @e[type=fox] slot.weapon.mainhand 1 skull 1 4)

Observed results:
The fox picks up an item that can be put on (in the slots for armor) and dresses it (you won't see the item in the fox's mouth until you use the command).

Expected results:
All items that the fox picks up must be in the fox's mouth (you see an item in the fox's mouth).