Aha, that makes sense. I figured it was something to do with the radiation. So the mirror world only exists where it could be seen through the mirror from the real world? That's a concept that could be developed further in some cool ways. It would mean, for instance, that a pillar or other large object might cast a sort of "shadow" of impassable "un-space" behind it, if there's no angle from which you could see behind it in the mirror. There could be puzzles involving shifting a large movable piece of machinery or cart in the real world to place the un-space such that it removes a particular hazard from existence in the mirror world.
Or maybe the un-space isn't impassible but anything that enters it fades out of existence. If you stick your hand in, it disappears, but you can pull it back out and it's there again, but if anything entirely enters the region of un-space, it's gone forever, or gets booted back to the real world or something.