Currently, the item returned by the getHelmetItem, getChestplateItem, getLeggingsItem, and getBootsItem functions always match the item equipped in their specified inventory slot. When CPM is being used alongside Cosmetic Armor Reworked, this won't always accurately reflect what the player visually appears to wear - if they are wearing a diamond chestplate, but have a leather one equipped in their cosmetic slot, the getChestplateItem function will return the diamond chestplate rather than the leather one. This is also the case if the player is wearing armour but has set it to be invisible.
It would be nice if compatibility could be added so that models will respond to what their player visually appears to be wearing, rather than what they actually have equipped. The mod's page says it has an API, which could be useful. For cases where a model creator might genuinely need to check what armour items are taking effect, rather than which ones are being rendered, entity.getInventoryItem() on slots 103, 102, 101, and 100 might be a good alternative.