This isn't just a nox issue and is far from a new issue - its been happening since detectors were first invented. I've experienced something similar with all of my detectors. CTX, Excal, CZ21, and Whites DF. I can get objects in certain positions, like on the side/edge and they won't read or depth will be severely diminished. I can't tell you how many times I have digging with all of these machines and all of a sudden can no longer pick up a signal until I sweep the sand around to get it again.
I was able to hit a small women's gold wedding band at 8 inches from the side in an air test with iron bias on 0 but as soon as I moved up the iron bias that depth was gone and on the highest setting couldn't even detect it. These results changed too based on my emf, iron with object, and recovery speed.