LVGL slider orientation with bg_images is not working

[ToriBox]

I would like to have the slider with images in vertical mode. For one strange reason it will not automatically rotate to vertical mode as a regular slider will do.

Any suggestions are welcome.

lvgl:
  pages:
  - id: Screen1
    scrollable: false
    widgets:
    - obj:
        id: screen_center
        align: CENTER
        width: 300
        height: 300
        widgets:
        - slider:
            id: my_vert_slider
            x: 20
            y: 50
            width: 10       # Height > Width sets it VERTICAL
            height: 220
            min_value: 0
            max_value: 40
            value: 26
        - slider:
            id: thermo_slider
            x: 50
            y: 50
            width: 10
            height: 250     # Height > Width should set it to VERTICAL!!!
            min_value: 0
            max_value: 40
            value: 26
            knob:
              bg_color: 0xFFFFFF
              bg_opa: 0
            main:
              bg_image_src: thermometer_slider_leeg
              bg_opa: 1.0
            indicator:
              bg_image_src: thermometer_slider_blauw
              bg_opa: 1.0