Your helmet appears unenchanted when in F5 mode whilst looking slightly upwards.
Steps to Reproduce:
1. Enchant a helmet with any enchantment. (I'd recommend using a netherite helmet as it is more noticeable when enchanted).
2. Equip the helmet and make sure that you are holding nothing in your hand.
3. Press F5 once and look slighty upwards and the helmet will appear as if its unenchanted.
The helmet appears unenchanted when viewing for a certain angle.
The helmet should always appear enchanted as it has enchantments on it.
I've attached two images. In one of them, you can see that I have an enchanted helmet equipped. In the other, you can see how when looking slightly upwards, the helmet appears unenchanted.
This work with any type of helmet, with any type of enchantment. This does not work with a chestplate, leggings or boots.