What is the mPower version installed on the MTCDT?
Is it different from what you have used previously?
On the LoRaWAN > Devices page the Session table has a details button for each session. The Session details popup shows the session keys.
What is the RSSI/SNR of the received packets. It is possible to receive packets on adjacent channels when devices are close to the gateway. Then the Conduit may try to respond on the wrong downlink channel.
I will be good to confirm with the device developer which channels are being used.
Can you get the session keys from the device?
Are there packet_recv messages after the packet_sent or is the device sending only Join Requests?
Is the device receiving the Join Accept?