That's correct, the ack will only be sent with the next uplink Where is a gotcha (at least it was to me): once the data is received, typically LoPy goes to deepsleep. Once it does, it forgets the previous session and starts the new one without confirmation. Indeed this was a problem, but from the upcoming firmware release on it won't - we now save the pending ack status of downlinks as well when using nvram_save(), so if you use nvram_restore after restarting from deepsleep the LoPy will know to send the acknowledgement with the next uplink. This is already in our internal code and will be public on the next release.

Thanks Meirssv for posting a solution. Had the same problem. Funny thing is that TTN only repeats the downlink payload on my LoPy module and not on my Microchip RN2483 module. It's like a downlink handshake from LoPy is missing, so TTN repeats queueing the payload. Can't find any documentation on downlink acknowledgements though. Anyway it works now thanks to your reply.