Custom Model Data can be used to define any block or items texture to display a certain way on the players head, except for helmets, whose vanilla models always take priority. This is a problem because if you want to use unique properties of helmets (such as the turtle shell's water breathing) with custom models, you can't.

Steps To Replicate:

• Install the Resource Pack,
• Run The Following Commands:

(Example of correct behavior on an item)

/give @s minecraft:stick[minecraft:custom_model_data={floats:[1]},minecraft:equippable={slot:head}]

(Example of correct behavior on a block)

/give @s minecraft:carved_pumpkin[minecraft:custom_model_data={floats:[1]}]

(Example of incorrect behavior on a helmet)

/give @s minecraft:diamond_helmet[minecraft:custom_model_data={floats:[1]}]

The correct behavior should be that defining custom model data to how a helmet's custom model looks on the head should overwrite the vanilla model on the head, like all other blocks and items, especially with the new display context system.