I'm assuming most closed helmets that don't show your face remove your facial hair and smush your face so it doesn't clip with the helmet model. While open helmets don't mess with your face.
Well the Spangenhelm is an exception, despite the nasal guard being almost identical to the viking helmet, it still mushes your nose back into your skull so everyone can see how stupid you look.
Please devs hotfix it I'm literally shaking and crying.