Using the item_model component doesn’t override helmet models. This is also an issue with resource packs.
Steps to reproduce: /give @s minecraft:iron_helmet[minecraft:item_model="minecraft:cow_spawn_egg"]
equip it
Observed result: Iron helmet model is rendered on the player’s head
Expected result: Cow spawn egg is rendered on the player’s head
Equipment models are controlled separately with the minecraft:equippable component.