my 2nd monitor kept deviating from those settings by going dim like it was about to go to sleep and only after i open the widget and joggle the slider would it go back to the proper setting (my primary monitor didn’t even have a slider).
the whole feature seems doughy and incomplete, so just disabled it.
be sure to set the brightness levels first because after this change the sliders will be gone… or add the package for a separate gui control
from my notes
# to keep brightness levels from changing, disable brightness control slider in system tray
systemctl --user edit plasma-powerdevil.service
# add these two lines to the file that opens
[Service]
Environment=POWERDEVIL_NO_DDCUTIL=1
# save and logout
# to reverse, just delete the directory created at
~/.config/systemd/user/plasma-powerdevil.service.d
# add this package for stand alone GUI control of monitor settings
gddccontrol