Got it to work 😄. The issue was the M5.begin call. I was passing true to DisplayEnable, but I guess there is some collision, so the device can't drive the strip and the matrix display at a same time. For my project the strip alone is good enough, so disabling the display is not a problem.
#include <Arduino.h>
#include "M5Atom.h"
#include "Adafruit_NeoPixel.h"
#define PIN 32
#define NUMPIXELS 10
#define DELAYVAL 500
Adafruit_NeoPixel pixels(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);
void setup() {
M5.begin(true, true, false);
pixels.setBrightness(255);
pixels.begin();
Serial.println("\nGroveLedStickTest\n");
}
void loop() {
pixels.clear();
for (int i = 0; i < NUMPIXELS; i++) {
pixels.setPixelColor(i, pixels.Color(255, 0, 255));
pixels.show();
delay(DELAYVAL);
}
}