I applaud your sense of safety. Usually the peel-off that covers adhesive is some sort of waxed paper or plastic. I'm not sure the UTH will get hot enough to set
fire to it, though it would dry out in time & become more likely as time goes on. It also occurs to me that the cover on the adhesive is going to act as insulation,
allowing LESS heat to rise into the enclosure, and that also makes it more likely to overheat. Not sure what to advise you, I've never used UTH with adhesive &
it sounds like yours is really designed to only be used once the backing is peeled, which is a real hassle: if it doesn't detach easily, the UTH itself will be damaged
if you ever try to remove it, & then you'll need to buy some more, which was perhaps their intention when they did it this way?