UV light can bounce off some surfaces, and depending on the style of even a full face mask it's possible for UV to bounce in from behind- been there done that- my mask is said to be full face, but it's not totally enclosed so when I do a bit of welding I now use a tea towel to cover back of head, and *loosely* cover gaps at side which are there to stop it steaming up inside. those reactive masks often have adjustment for both how fast they react, and how dark they go. If the lens is intact and with proviso above it should be stopping direct UV (Richard Feynman commented on how even plain automotive windscreen glass would stop UV, when talking about how he chose to ignore advice and look directly at the blast of the first test atomic bomb from a car, reasoning it would stop the dangerous UV) But if it's not going dark enough, or quickly enough after spark is struck, maybe a significant amount of IR could be passing through.either during welding or each time you start up. you might not get conjunctivitis(sore eyes) from IR. You haven't said if the mask is a new one bought for your use, or if it's been loaned to you. there's a possibility your skin could be reacting to something that's inside the mask. The smell of new plastic/rubber is down to chemicals like solvents, which some people's skin will react to. Old masks that have been worn by others could have all kind of stuff that your skin mightn't like, including salt from sweat, and bacteria that your skin just ain't used to. or even some kind of cleaning agent. Could even be down to sweat/condensation/general damp if your face has been inside a mask that has poor air flow for a long time.