Hmmmm... I thought Invisible Air was a Noble Phantasm's of Saber's, not a function of the Grail. Wasn't it made by Merlin? I mean, if the Grail went out of its way to protect every incredibly obvious and identifiable NP, then Gae Bolg would've been invisible too, which I'm pretty sure would've legit made Lancer the most broken Servant in that war.