if you look into Power.cpp you'll see that the current already is set to about 400 mA:
https://github.com/m5stack/M5Stack/blob/master/src/utility/Power.cpp

void POWER::begin() {
  uint8_t data;
  //Initial I2C 
  Wire.begin(21, 22);
  // 450ma 
  setVinMaxCurrent(CURRENT_400MA);

are you calling M5.Power.begin() in your setup code?
Or maybe the current has been reduced due the the battery being close to full?

Cheers
Felix