I have the same vkorn add-on you do. I have an extremely similar issue to you.
Communication has been very one way, state updates from smartthings have been good, HA, reflects them almost instantly. Try to turn the light off in HA and MQTT fires the off cmd to the bridge, but nothing happens except HA flipping the status back to “off” a few seconds later.
info: Subscribing to smartthings/Dining Hall/switch/cmd
info: Incoming message from SmartThings: smartthings/Dining Hall/switch/state = on
info: Incoming message from SmartThings: smartthings/Dining Hall/switch/state = off
info: Incoming message from SmartThings: smartthings/Dining Hall/switch/state = on
info: Incoming message from MQTT: smartthings/Dining Hall/switch/cmd = off
info: Incoming message from MQTT: smartthings/Dining Hall/switch/cmd = off
EDIT:
It’s important to use the correct MAC
MAC address is everything in smartthings, My pi was connected via wi-fi, smartthings wanted to see the wlan0 MAC address of my PI, not the eth0 address. Good Luck!