Brightness in scenes does not work with template lights

Configuration
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1

Dear all, I have a lamp hanging over my dining table with four ikea trädfri bulps (for dimming purposes) and I also have a shelly switch in the wall behind the haptic switch. To get both bulbs and switch working togehter I have configured a template light. This ensures the switch is turned on if necessary together with the bulbs and only turns of the bulbs if the switch is pressed again. This works really like a charm, also diming the lights is possible via the appropriate cards in the frontend.

What however not works is setting the template light to a certain light level / brigthness when using scenes. Neither when using scene.apply within an automation nor when manually activating a scene. Is this a known problem? How can I tackle this??

Please see below my code snippets:

Template Light

  - platform: template
    lights:
      diningroom_lamp:
        friendly_name: "Esszimmerlampe (unified)"
        level_template: "{{ state_attr('light.bulbs_diningroom', 'brightness')|int }}"
        value_template: "{{ is_state('switch.light_diningroom', 'on') and is_state('light.bulbs_diningroom', 'on')}}"
        color_template: "{{ state_attr('light.bulbs_diningroom', 'rgb_color') | float}}"
        turn_on:
          - service: homeassistant.turn_on
            target:
              entity_id: switch.light_diningroom
          - service: light.turn_on
            target:
              entity_id: light.bulbs_diningroom
            data:
             transition: 1
        turn_off:
          service: light.turn_off
          target:
            entity_id: light.bulbs_diningroom
          data:
            transition: 1
        set_level:
          - service: homeassistant.turn_on
            target:
              entity_id: switch.light_diningroom        
          - service: light.turn_on
            target:
              entity_id: light.bulbs_diningroom
            data_template:
              brightness: "{{ brightness }}"
        set_temperature:
          service: light.turn_on
          target:
            entity_id: light.bulbs_diningroom
          data_template:
            color_temp: "{{ color_temp }}"

Scene:

- id: livingroom_lunch
  name: Essen
  icon: mdi:silverware
  entities:
    light.diningroom_lamp: 
      state: 'on'
      brightness_pct: 50
      color_temp: 300    
    light.led_tv_panel: 'off'
    light.livingroom_corner:
      state: 'on'
      brightness_pct: 1
    light.stonewall_livingroom: 'off'
    light.livingroom_spots:
      state: 'on'
      brightness_pct: 20
      color_temp: 500
    media_player.samsung_qled: 'off'

Code behind my button in the lovelace UI to activate the scene

                  - type: button
                    entity: scene.livingroom_eat
                    show_name: false
                    tap_action:
                      action: call-service
                      service: scene.turn_on
                      service_data:
                        transition: 2
                        entity_id: scene.livingroom_eat
                      target: {}
                    hold_action:
                      action: none