LORAMAC_EVENT_INFO_STATUS_ADDRESS_FAIL on expected downlink

[dmeehan1968]

Network Server: TTN
Auth: OTAA
Region: AS923
ADR: Enabled

I've been seeing these occasionally when I have an expected downlink (device sends a LinkCheckReq along with a payload which solicits the application server to schedule a downlink which is received on the subsequent uplink (LinkCheckAns comes on the initial uplink). The failure sometimes comes on the downlink containing the LinkCheckAns, and sometimes on the subsequent.

The device sends uplinks which aren't expected to solicit downlinks and all of these get indicated OK. Periodically the device sends a payload also with LinkCheckReq to validate the network connection and solicit config data from the application server. The sequence might look like:

1. TX (payload + LinkCheckReq)
2. RX (LinkCheckAns)
3. TX (repeat payload, no Link check if received at 2)
4. RX (solicited payload from 1)
5. TX (confirmed message, payload)
6. RX (solicited payload from 3)

The LORAMAC_EVENT_INFO_STATUS_ADDRESS_FAIL appears on the RX confirmation and the expected downlink payload is lost, as is the LinkCheckAns). Sometimes the error is reported on 2 and sometimes on 4. In both cases, a further TX/RX exchange in necessary, so far that's always recovered but obviously wastes airtime.

Looking at the code suggests that this is a mismatch in DevAddr for the packet received in the RX window, which seems unlikely. In the test environment, there is only a single device active on that region (the device in question).

The device has been tested using Loriot without encountering this error, only with TTN as the NS, and only on AS923.

I'm trying to work out if this could be an NS or LoRaMac issue. Anyone seen similar or can shed anymore light?