Curious, does ddcutil also not work correctly?
I am on arch, and one of the latest KDE updates regressed back to wonky behavior of brightness/backlight.
If you have ddcutil
# check that your screen is detected:
ddcutil detect
# make sure Brightness is a detected capability:
ddcutil capabilities | grep "Feature: 10"
# You can compare the setting in ddcutil vs what the percentage is on your deck
# Check the current brightness level:
ddcutil getvcp 10
# Set brightness (in %, ie this example is 70%):
ddcutil setvcp 10 70