@jimw Please try this code.
Serial speed 115200
and the firmware (1.2.3-en) for micropython
It is not for Arduino.
#include <M5Stack.h>
int servoPin = 5;
void servoSet(int Angle, int Time) {
  int pulseWidth;
  int TimeCount;
  TimeCount = Time / 20;
  for (int i = 0; i <= TimeCount; i++) {
    pulseWidth = map(Angle, 0, 180, 544, 2400);
    digitalWrite(servoPin, HIGH);
    delayMicroseconds(pulseWidth);
    digitalWrite(servoPin, LOW);
    delayMicroseconds(20000 - pulseWidth);
  }
}
void setup() {
  M5.begin();
  M5.Lcd.printf("M5Stack Servo test:\n");
  pinMode(servoPin, OUTPUT);
}
void loop() {
  M5.Lcd.printf("Angle 45deg\n");
  Serial.println("Angle 45deg");
  servoSet(45, 1000); //45deg
  M5.Lcd.printf("Angle 135deg\n");
  Serial.println("Angle 135deg");
  servoSet(135, 1000); //135deg
}