Hi
Using Home assistant, and Optimistic set to false in the mysensors config, the switch in homeassistant would turn the relay on, but in the view in homeassistant the flip switch jumped off straight after switching on. It was solved by adding the following line to the sketch, ensuring that hassio knows that the actuator actually have received the command. Not sure if this is a good way of doing it, but it seems to work for me.
send(msg.set(state)); // Send new state and request ack back
in:
void receive(const MyMessage &message) {
// We only expect one type of message from controller. But we better check anyway.
if (message.isAck()) {
Serial.println("This is an ack from gateway");
}

if (message.type == V_LIGHT) {
// Change relay state
state = message.getBool();
digitalWrite(RELAY_PIN, state?RELAY_ON:RELAY_OFF);
// Store state in eeprom
saveState(CHILD_ID, state);

 // Write some debug info
 Serial.print("Incoming change for sensor:");
 Serial.print(message.sensor);
 Serial.print(", New status: ");
 Serial.println(message.getBool());
 send(msg.set(state)); // Send new state and request ack back

}
}

Thanks