Sounds exactly like my experience. Installer dumbfounded, "never had it happen before", all the same explanations about electronically calibrated equipment making it impossible to screw up, etc..... And, it isn't water "pushing the smell out". The stuff is water resistant, as you note. It eventually got around to tearing stuff out, and we found elongated weird gooey spots with a few globs. No reasonable explanation was ever determined. We don't know why it went bad, but it went bad.