My code always gets exception decoder message and resets a few ms after MQTT connection is done successfully.
My connectMqtt function:
void MyClass::connectMqtt(PubSubClient client)
{
  // Loop until we're reconnected
  int attempts = 0;
  bool connected = false;
  while (!connected && attempts < 5)
  {
    std::string clientId = _my_device.getBarcode();
    // Attempt to connect
    if (client.connect(clientId.c_str(), _my_device.getMqttUser(), _my_device.getMqttPassword()))
    {
      Serial.println(F("MQTT connected"));
      connected = true;
      //Subscribe to config topic of the device
      //TODO not done yet
    }
    else
    {
      Serial.print(F("failed, rc="));
      Serial.print(client.state());
      Serial.println(F(" try again in 5 seconds"));
      attempts++;
      // Wait 5 seconds before retrying
      delay(5000);
    }
  }
  Serial.println(F("Done."));
}
Exception decoder mesage starts by:
Exception (3):
epc1=0x40100691 epc2=0x00000000 epc3=0x00000000 excvaddr=0x40032468 depc=0x00000000
 
                        
As @Juraj said:
The solution is passing a reference to the function, not a copy