Ah my bad, thanks for the clarification. Does Chirpstack then “translate” the message into the “<region_prefix>/gateway/<gateway_id>/command/down” topic format which the Gateway Bridge subscribes to?
As for the original post could it be that the payload is incorrect? From the link @bconway posted it appears your MQTT payload:
{“devaddr”: “00169871”, “data”: “MDI=”", “port”:6}
is missing important information like devEui and Confirmed.