So the loop function attempts to connect to WiFi and load the data, then disconnects; then either updates the display or doesn't; then attempts to shutdown. If the M5paper is on power, then the M5.shutdown command has no effect, so the code then calls esp_light_sleep_start.

When it wakes up, if the unit is on battery (so it's a complete start-up) it all works as normal. If it is on power (so it's just resuming after 'light sleep') then it fails to connect to WiFi.

If I replace the call to esp_light_sleep_start with a call to delay for the same period, then it connects repeatedly without issue.

A summary of the code (this is using the WiFiMulti (ESP8266WiFiMulti) library by Markus Sattler, dated 16.05.2015, to support working across several locations with different WiFi availability):

WiFiMulti wifiMulti;

boolean connectToWifi()
{
  uint8_t tRes;

  for (int i = 0; i < NUMWIFINETWORKS; i++) {
    wifiMulti.addAP(aWifiNetworks[i], aWifiPasswords[i]);
  }

  Serial.printf("Connecting Wifi... trying %d networks\n", NUMWIFINETWORKS);
  if ((tRes = wifiMulti.run()) == WL_CONNECTED) {
    Serial.print("WiFi connected ");
    Serial.println(WiFi.SSID());
    Serial.print("IP address: ");
    Serial.println(WiFi.localIP());
    return true;
  } else {
    Serial.printf("Failed to connnect to WiFi: %d.\n", tRes);
  }

  return false;
}

void disconnectFromWifi()
{
  Serial.println("disconnecting from WiFi");
  WiFi.disconnect();
}

Evidently something in the course of esp_light_sleep_start 'breaks' something that the WiFi needs, so I need to reset it. But I don't know what, or how I can reset it!

Can anyone shed any light?