If you create an item using the Holiday Creator Features in an add-on, you have the option to set the max_stack_size. If you also add the wearable and armor components, you can put it in the designated armor slot. Thus, it is possible to create stackable armor pieces. You can equip armor by clicking on the armor slot with the item, shift clicking from the inventory, or by using the item when it is in your main hand. Both clicking on the slot and shift clicking from the inventory will deposit 1 of the armor pieces in the proper slot and leave the rest in your inventory if the item has a max_stack_size greater than 1. However, if you try equipping from the mainhand by using the item, it removes 1 from the stack like you would expect, but it also duplicates the stack by putting more than 1 item in the armor slot.
For example, it you had a 64 stack of a custom helmet, trying to equip it by using the item would leave 63 items in your hotbar (as expected) but would also place 64 in the helmet armor slot, which should not happen.
Mob Skulls from the vanilla Bedrock game can be placed manually and shift clicked like all other armor items, but they cannot be equipped by using the item, which prevents this bug from affecting them. For this bug to be fixed, there should be an option in the minecraft:wearable component, like "equip_on_use" which by default is set to true and maintains current behavior, but when set to false will act like player heads, not allowing the player to use the item.