They may also not want to release that hairstyle because, depending on how averse they are to giving 'old spoilers', they avoided showing him not wearing his armour for a very long time - even in the lorebook which includes a story summary to the end of the Dragonsong arc, though that might be due to a lack of decent screenshots. (I found it quite frustrating throughout Heavensward, having this major companion character but not really knowing what they look like! I wish they'd shown his face properly earlier. The 'campfire scene' before visiting Zenith would have been a good opportunity.)
Though I'm not sure that it *isn't* a unique face? I played around a bit in the creator but didn't feel like I could get an exact match. Also I'm not sure he looks 100% the same 'unmasked' compared to the few glimpses you get of his face under the armour. (My character is short enough that I could see under the visor if he was standing around in-game and I got up really close to him.) Particularly the very dark sunken look around his eyes, though that's a justifiable change after he's spent several weeks? months? in the wilderness possessed by a vengeful dragon spirit. Combined with a few times that they've avoided showing his face even after you've seen it - eg. the closing cutscenes for Dragonsong, where he's only shown from behind or from the chest down, and
this side story where they've specifically made a character design for the illustrations and *still* avoid his face, when they could just use the model that already exists - I have to wonder if there was a point where they were redesigning him and told animators to avoid showing his face in cutscenes until they finalised it.
(I assume he does eventually turn up in game - have just finished the Stormblood main game and they just keep teasing me at this point! If/when he finally reappears and is standing around in the game I'll be interested to take a comparative screenshot of his face from the same angle as I could see under his armour.)