The regulation for oxygen use and TSO for the mask are two completely different things. The mask is certified for use up to FL410 cabin pressure altitude, not aircraft altitude. Unless of course one experiences explosive decompression and “stays” cruising at FL410 (nobody would), there should be no mask functionality issues and the mask should continue to operate as it is designed.
When everything is working fine and you are crusing at FL450 and one crewmember is wearing/using the mask per the regulation, the mask is being used at a cabin altitude of around 8000’ at max “delta P”. Thus well within its design TSO operating parameters.