Brightnesss control with ddcutil for external monitors seems to be wonky again.
Check your monitor settings (on the monitor, not in the Plasma system settings) if you can find an option called somethin like DDC or DDC/CI and disable that.
Redo your brightness settings for the monitor (on the monitor, not in the Plasma system settings) and reboot.
You can also tell the powerdevil service to stop using ddcutil:
systemctl --user edit plasma-powerdevil.service
# Add this:
[Service]
Environment="POWERDEVIL_NO_DDCUTIL=1"
# Restart the service:
systemctl --user restart plasma-powerdevil.service
Redo your brightness settings for the monitor (on the monitor, not in the Plasma system settings) and reboot.
I did both for my desktop and notebook since ddcutil has been causing troubles on Plasma a few times in the past already.
On my desktop I never want my brightness to be changed by software anyway and on my notebook all brightness controls and power saving functions regarding the display seem to be working fine without powerdevil using ddcutil.