I’m using the Smartthings Mosquitto MQTT-Bridge (in a docker) quite successfully. This weekend, I got adventurous and decided to add thermostats to the mix. I can see and control the Nexia XL824 thermostats via Smartthings through a third party smart app and device handler someone developed. After adding the thermostats to the MQTT Bridge, I can see all the info flowing through the events.log. Also, when I make any changes to the Smartthings app, they are immediately displayed on the HA front end. However, if I try to make the same changes in HA, nothing happens on the Smartthings end. This is surprising because everything else going from HA through the bridge - lights, switches, etc - is seamless. Couple of things to note: the temperature numbers go up or down, depending on which arrow I click, but nothing changes in smartthings. However, when trying to set the operation mode or the fan mode, I can’t select anything in the pull down menu. It’s stuck on the mode it’s receiving from smartthings. In this case, that would be “Auto” for both. Any help would be appreciated. The first picture below is pulled out of the events.log…this isn’t all the thermostat data, but just meant to show that it’s coming into the event.log. The second shot is of the downstairs thermostat. 
Hass 0.58 installed on ubuntu 16.04 virtual machine
# Upstairs Thermostat
- platform: mqtt
name: "Upstairs Thermostat"
optimistic: true
retain: true
qos: 1
modes:
- "off"
- heat
- cool
- auto
fan_modes:
- auto
- "on"
- circulate
current_temperature_topic: "smartthings/Upstairs Thermostat/temperature"
temperature_state_topic: "smartthings/Upstairs Thermostat/thermostatSetpoint"
temperature_command_topic: "smartthings/Upstairs Thermostat/thermostatSetpoint/set"
mode_state_topic: "smartthings/Upstairs Thermostat/thermostatMode"
mode_command_topic: "smartthings/Upstairs Thermostat/thermostatMode/set"
fan_mode_state_topic: "smartthings/Upstairs Thermostat/thermostatFanMode"
fan_mode_command_topic: "smartthings/Upstairs Thermostat/thermostatFanMode/set"
# Downstairs Thermostat
- platform: mqtt
name: "Downstairs Thermostat"
optimistic: true
retain: true
qos: 1
modes:
- "off"
- heat
- cool
- auto
fan_modes:
- auto
- "on"
- circulate
current_temperature_topic: "smartthings/Downstairs Thermostat/temperature"
temperature_state_topic: "smartthings/Downstairs Thermostat/thermostatSetpoint"
temperature_command_topic: "smartthings/Downstairs Thermostat/thermostatSetpoint/set"
mode_state_topic: "smartthings/Downstairs Thermostat/thermostatMode"
mode_command_topic: "smartthings/Downstairs Thermostat/thermostatMode/set"
fan_mode_state_topic: "smartthings/Downstairs Thermostat/thermostatFanMode"
fan_mode_command_topic: "smartthings/Downstairs Thermostat/thermostatFanMode/set"