Equippable items with item_model parameter do not inherit the model of item when worn.

Get an item which is both equippable and has the item model of an actual wearable item:

/give @s clock[minecraft:equippable={"slot":"head"},minecraft:item_model="zombie_head"]

Wear the item.
Expected result is the item to look like a worn zombie head on the head.
The result is the item is not recognized as its item model and seems different than a worn zombie head.