I own a M1X from the first run and the light starts to dim when the two 18650 are almost empty. My guess is that the M1X uses a buck-driver and a MC-E in 2S2P (=Vf ~ 6.5-7V) ... so there is no chance that the 4.2 volts max. of a single 18650 will work. Maybe it will be enough for a faint glow ...
I can´t say for sure that recent versions of the M1X use the same config, but it´s very likely (imho).