Face mask or face guard in late roman period

[Renatus]

Yes, I know, and that's the only occasion, and from the text of Ammianus it's also clear that this was meant to be quite the spectacle. Some parts of the description defy logic ("Thin circles of iron plates, fitted to the curves of their bodies, completely covered their limbs; so that whichever way they had to move their members, their garment fitted, so skilfully were the joinings made.") and seem to be sprouting from fantasy rather than an eye-witness account, which makes me doubt that the passage is 100% trustworthy.
So in fact we have no idea if this was a regular helmet, or perhaps added for dramatic purposes (I mean, even without a face mask, do cataphracts really look like metal statues??) or that we may see a Sassanid Persian or perhaps Ghassanid Syrian cavalry unit here? There's no telling.

Anyway, I'm not prepared to accept 'Late Roman masked cavalry helmets' as a common item, just on the basis of this single passage.

You seem to have overlooked Julian's panegyric to Constantius, already cited by Nathan, which gives you another example. I accept that there is a certain amount of rhetoric in Ammianus' description but there is no need to discount its basic accuracy. He describes Sassanian forces in similar terms (Amm. 24.2.10; 24.7.8; 25.1.12). I see no reason to doubt that the clibanarii mentioned by Ammianus were Roman. On the other hand, such units did not form an overly numerous element of the Late Roman army (only ten units are mentioned in the Notitia) and I don't think that there is any evidence to suggest that regular cavalry units were equipped with masked helmets (those cited in Julian's panegyric being almost certainly clibanarii), so I will go along with you in saying that they were not "a common item".