HI @scle,
I have done as you mentioned, I enabled the automatic reply, and it seems to be working since I see again the acknowledge in the gateway, so this confirms as well that th gateway works with acknowledge at least from its web interface to send data to motes.
You are right, the LoraWan does not tell much about how the acknowledge should be sent from the mote... On the other hand, concerning more to an implementation way, the TX has to implement as well a mechanisim to check after each transmission if something was sent to him right?