My mooshimeters have never met ios-devices, so i can’t really say if that is “normal” or just bad luck.
But, there is one thing, which does help in many problems with mooshimeter.. Have you tried new batteries? Even if battery status says that there is lot of percents left. “Idle” voltage on some batteries does not degrade linearly with the available capacity and might screw the results of indicators that only check voltage, like mooshimeters. So, even if there seems to be adequate voltage left, it might dip too much when trying to do something and crash the system.