Create (and thus Create Fabric) adds some armor pieces:

Pre-0.5.1 items:
Copper Diving Helmet (create:copper_diving_helmet)
Copper Backtank (create:copper_backtank)
Copper Diving Boots (create:copper_diving_boots)

0.5.1:
Netherite Diving Helmet (create:netherite_diving_helmet)
Netherite Backtank (create:netherite_backtank)
Netherite Diving Boots (create:netherite_diving_boots)

None of these are hidden by Cosmetic Armor, except when leather armor is placed in the cosmetic slot.

Oddly, also, the Netherite Diving Helmet (crafted with a Copper Diving Helmet and a Netherite Helmet or a Copper Diving Helmet and a Netherite Ingot) looks like a Netherite Helmet when placed in the cosmetic slot.

Same issue here