The masks that Martial Artists wore were not meant to protect their face or head, it was meant to protect their identity. Of course, Martial Apprentices got hit in the face and the head regularly thus the masks that Martial Apprentices wore were designed to survive the rigor of Apprentice-Realm combat. Of course, most masks needed heavy maintenance after rigorous extended combat.
Ordinary substances that couldn't withstand the rigors of Apprentice-Realm combat while the esoteric resources with the tensile strength to be able to completely endure Apprentice-Realm combat were strategic resources that also too scarce to be wasted on masks.
Thus, as well as masks were designed to survive and at the very least stay intact, when faced power and onslaught at effectively the pinnacle of the Apprentice Realm, they simply couldn't stay intact.
Rui hadn't even realized that his mask had fallen off until he felt the warm air make contact with his face.