ESP32 MQTT onMessage Callback never called

Other Hardware 3rd Party Boards
18

If I publish an empty message to $aws/things/chicken-door/shadow/name/door/get I receive the following message on $aws/things/chicken-door/shadow/name/door/get/accepted:

{
  "state": {
    "desired": {
      "welcome": "aws-iot"
    },
    "reported": {
      "welcome": "aws-iot",
      "status": "off",
      "value": "closed"
    }
  },
  "metadata": {
    "desired": {
      "welcome": {
        "timestamp": 1637421413
      }
    },
    "reported": {
      "welcome": {
        "timestamp": 1637421413
      },
      "status": {
        "timestamp": 1637735730
      },
      "value": {
        "timestamp": 1638447783
      }
    }
  },
  "version": 1012,
  "timestamp": 1638793997
}

That would be more than 256 (321bytes). So I set the buffersize to 512:

void setup()
{
  Serial.begin(115200);
  delay(5000);
  Serial.println();
  Serial.println();

  WiFi.setHostname(THINGNAME);
  WiFi.mode(WIFI_STA);
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
  connectToWiFi(String("Attempting to connect to SSID: ") + String(WIFI_SSID));

  net.setCACert(AWS_CERT_CA);
  net.setCertificate(AWS_CERT_CRT);
  net.setPrivateKey(AWS_CERT_PRIVATE);

  client.setServer(AWS_IOT_ENDPOINT, MQTT_PORT);
  client.setBufferSize(512);
  client.setCallback(messageReceived);

  connectToMqtt();
  getShadow();
}

Still no luck unfortunately.. here is the output:

Attempting to connect to SSID: dehei.local...ok!
MQTT connecting connected!
Requesting Shadow [$aws/things/chicken-door/shadow/name/door/get]: 
10658
15659
20660