Mqtt_template - what have I done wrong

Configuration
1

This is driving me nuts. I have an mqtt template that is ignoring mqtt incoming messages. My yaml is:

  - platform: mqtt_template
    name: 'wifi_dimmer'
    brightness: true
    command_topic: "/ha/dimmers"
    state_topic: "/ha/dimmers"
    command_on_template: >
      {"id": "lounge", "source": "ha"
      {%- if brightness is defined -%}
      , "brightness": {{ brightness | multiply (4) | int}}
      {%- endif -%}
      }
    command_off_template: '{"id": "lounge", "source": "ha", "brightness": 0}'
    brightness_template: '{{ value_json.brightness }}'
    optimistic: false

When I send the message below - I’d expect the brightness to set the the value in the message - however nothing changes at all.

{"id":"lounge","source":"lounge","brightness":16}

So - what have I done wrong?