This exact thing happened to me tonight, with an M10 and a Church Ugly battery box, and new batteries. Weirdly, if I unplugged the cable from the M10 mic in and replugged it, it seemed to fix itself, but then would fade again.
(That's all from monitoring it live; I haven't checked the waveform yet, as it's 1 am and I just got home.)
This has happened to me a couple of times before, and I blamed a dying battery at the time. But this one is just weird.