@brocaar should we raise a bug for this topic? probably 1 or 2 different topics.
fport is always null also if downlink ACK message is sent out successfully. I found other fport=null examples from other users in the forum (e.g. Device sends ConfirmedUp but no UnconfirmedDown is initiated)
So fport check here and at https://github.com/brocaar/chirpstack-network-server/blob/69ba076612497423362937db898b424544174205/internal/downlink/ack/ack.go#L499 are wrong and/or fport must be set elsewhere also on ACK messages