Lovelace - Icons not appearing on Android suddenly

In the past few days my Android tablets used as wall displays for HASS have stopped showing the icons in the title bar, the side bar and even in panels.

It just shows white space (transparent as it shows the header colour) but the icons still work (to switch panels, for example).

The icons appear on desktop browser (Chrome & Brave) but not on Android devices using Chrome, Brave or the Hass App.

An example of the ‘code’ follows - when icon is commented out the word ‘Lights’ displays, when not commented out, neither the icon or the text appears.

views:
# Header
  - title: Lights
    icon: 'mdi:lightbulb-on-outline'
    cards:
      - type: entities

The YAML code has not changed other than when I’m trying to get it to work by commenting ‘icon’ out!

Has anyone any ideas why this is and what I can do to fix it, please?

Thanks.

Richard