I have been struggling to get this working over the past week, and have finally figured it out. Posting in case anyone else runs into this issue.
The problem was that I could see the SmartThings switch activity in the smartthings-mqtt-bridge logs, but could not control the SmartThings switch via my HA switch. The HA switch would not react to incoming messages or post to SmartThings.
I was able to see that SmartThings was sending the MQTT states as “smartthings/Living room Lights/switch/state”; however in my HomeAssistant configuration, I had specified the state_topic as “smartthings/Living room Lights/switch” which is consistent with all of the documentation I have been able to find. Changing the state_topic to “smartthings/Living room Lights/switch/state” and the command_topic to “smartthings/Living room Lights/switch/cmd” has finally bridged the gap between my HA switch and SmartThings.
Everything now works as expected. Moral of the story: don’t assume your devices are publishing the same messages as everyone else