Using Qt 6.7.0, KF6 from git along with Plasma6 from Git, this certainly used to work

journalctl -b | grep -i "bright\|keyboard"
Apr 24 12:31:12 axion.fireburn.co.uk kernel: atkbd serio0: Failed to deactivate keyboard on isa0060/serio0
Apr 24 12:31:12 axion.fireburn.co.uk kernel: asus 0003:0B05:1866.0002: input,hiddev0,hidraw1: USB HID v1.10 Keyboard [ASUSTek Computer Inc. N-KEY Device] on usb-0000:08:00.3-3/input0
Apr 24 12:31:12 axion.fireburn.co.uk kernel: atkbd serio0: Failed to enable keyboard on isa0060/serio0
Apr 24 12:31:12 axion.fireburn.co.uk kernel: input: AT Translated Set 2 keyboard as /devices/platform/i8042/serio0/input/input4
Apr 24 12:31:12 axion.fireburn.co.uk systemd[1]: Starting Load/Save Screen Backlight Brightness of backlight:amdgpu_bl1...
Apr 24 12:31:12 axion.fireburn.co.uk systemd[1]: Starting Load/Save Screen Backlight Brightness of leds:asus::kbd_backlight...
Apr 24 12:31:12 axion.fireburn.co.uk systemd[1]: Finished Load/Save Screen Backlight Brightness of backlight:amdgpu_bl1.
Apr 24 12:31:12 axion.fireburn.co.uk systemd[1]: Finished Load/Save Screen Backlight Brightness of leds:asus::kbd_backlight.
Apr 24 12:31:12 axion.fireburn.co.uk systemd-logind[436]: Watching system buttons on /dev/input/event10 (Asus Keyboard)
Apr 24 12:31:12 axion.fireburn.co.uk systemd-logind[436]: Watching system buttons on /dev/input/event13 (AT Translated Set 2 keyboard)
Apr 24 12:31:15 axion.fireburn.co.uk kwin_wayland_wrapper[660]: The XKEYBOARD keymap compiler (xkbcomp) reports:
Apr 24 12:31:18 axion.fireburn.co.uk plasmashell[724]: error getting max keyboard brightness via dbus QDBusError("org.freedesktop.DBus.Error.UnknownObject", "No such object path '/org/kde/Solid/PowerManagement/Actions/KeyboardBrightnessControl'")
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]: file:///usr/share/plasma/plasmoids/org.kde.plasma.brightness/contents/ui/PopupDialog.qml:21:18: QML ScrollView: ScrollView only supports Flickable types as its contentItem
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]: QQmlExpression: Expression file:///usr/share/plasma/plasmoids/org.kde.plasma.brightness/contents/ui/KeyboardColorItem.qml:26:5 depends on non-NOTIFYable properties:
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]:     KeyboardColorControl::supported
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]: QQmlExpression: Expression file:///usr/share/plasma/plasmoids/org.kde.plasma.brightness/contents/ui/KeyboardColorItem.qml:77:21 depends on non-NOTIFYable properties:
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]:     KeyboardColorControl::enabled
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]: QQmlExpression: Expression file:///usr/share/plasma/plasmoids/org.kde.plasma.brightness/contents/ui/KeyboardColorItem.qml:64:21 depends on non-NOTIFYable properties:
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]:     KeyboardColorControl::enabled
Apr 24 12:31:24 axion.fireburn.co.uk plasmashell[724]: file:///usr/share/plasma/plasmoids/org.kde.plasma.brightness/contents/ui/KeyboardColorItem.qml:67:21: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }