I don’t think a node ever needs to reply immediately. But indeed, like for a confirmed downlink, it needs to store the fact that a confirmation was requested, so it can set the ACK bit in the next uplink. Even if that uplink is days later. When not doing that, then TTN will be required to send the downlink again.
(So I guess mac save indeed does not suffice there. But I even don’t see the frame counters mentioned above, so maybe though the documentation is outdated, see below.)mac save is just not the right command at all, or
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