SUMMARY
Sometimes powerdevil stops reacting to brightness change requests, though Plasma shows brightness bar when using relevant key combos or using "Brightness and Color" widget. 


STEPS TO REPRODUCE
Not sure what triggers that. Can be related to suspending/resuming the system.
In any case, try to change brightness with KEY_BRIGHTNESSUP/KEY_BRIGHTNESSDOWN.

OBSERVED RESULT
Brightness didn't change, though Plasma reports changed brightness

EXPECTED RESULT
Brightness should change

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20251127
KDE Plasma Version: 6.5.3
KDE Frameworks Version: 6.20.0
Qt Version: 6.10.1
Kernel Version: 6.18.0-5.g6329d00-default (64-bit)
Graphics Platform: Wayland
Processors: 16 × 13th Gen Intel® Core™ i7-1360P
Memory: 16 GiB of RAM (14.8 GiB usable)
Graphics Processor: Intel® Graphics
Manufacturer: LENOVO
Product Name: 21HEA021CD
System Version: ThinkPad T14 Gen 4

ADDITIONAL INFORMATION
It is certainly not a kernel bug, since brightnessctl works as expected.
Moreover, I can not restart powerdevil: systemctl --user restart plasma-powerdevil.service hangs on service startup stage.

systemctl --user status plasma-powerdevil also shows this:
Dec 06 01:56:31 eternity systemd[2460]: Started Powerdevil.
Dec 06 01:56:31 eternity org_kde_powerdevil[2809]: Handle button events action could not check for screen configuration
Dec 06 01:56:31 eternity org_kde_powerdevil[2809]: Failed to register with host portal QDBusError("org.freedesktop.portal.Error.Failed", "Could not register app ID: App info not found for 'org.kde.org_kde_powerdevil'")
Dec 06 01:56:31 eternity org_kde_powerdevil[2809]: Failed to register with host portal QDBusError("org.freedesktop.portal.Error.Failed", "Could not register app ID: Connection already associated with an application ID")
Dec 06 22:58:22 eternity org_kde_powerdevil[2809]: "Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken."
Dec 06 22:58:22 eternity org_kde_powerdevil[2809]: [BacklightBrightness]: Failed to set screen brightness "DBus Backend error: could not contact the helper. Connection error: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.. Message error: "