Capslock / Numlock indicator not work in KDE (& sddm)

Hi,

I'm using Debian testing and run KDE Plasma 5 (Framework 5.62.0, Qt 5.12.5, xcb window system, Application 19.08.1, kde-plasma-desktop version 5:104).  Originally the indicator worked: in sddm login screen it would show warning when Capslock is ON; after logging into Plasma session a small icon would show beside the system tray when Capslock is ON.  However from some day it didn't work anymore.

The current status of my system is:

• In sddm login screen, the Capslock ON warning *always* shows.
• In Plasma the keyboard indicator plasmoids (org.kde.plasma.keyboardindicator) shows nothing no matter what the capslock/numlock status is.
• Using xset -q I can get *correct* Capslock/Numlock status.  Using xev I can see events of Capslock key being pressed and released.

It looks like no problem in X window level, but the status does not pass to sddm/plasma correctly. 

Any idea or suggestion for finding or fixing the problem?

Regards, Franklin