If I configure powerdevil/solid to handle the screen brightness shortcut in system settings, it works perfectly, but nothing happens when KDED is configured to handle it. I can use the power management plasmoid/applet to control brightness also. dbus-monitor doesn't show anything when I try to adjust the brightness with these keys when kglobalaccel is configured to use kded, but it does show some output when using powerdevil/solid. KDED is also configured to handle Meta+P for external displays, and that shortcut works.
Same problem Manjaro Linux Plasma 5.12. All other fn+Function keys work (sound, touchpad, keyboard brightness). Screen brightness and scroll lock do not. https://forum.manjaro.org/t/thinkpad-brightness-adjustment-by-keyboard-not-working-anymore/40460
*** This bug has been marked as a duplicate of bug 389845 ***