By new upower powerdevil overflow brightness calculation and it cannot set it over 30%.
https://bugs.kde.org/show_bug.cgi?id=454161
https://build.opensuse.org/package/view_file/home:lemmy04:branches:KDE:Frameworks5/powerdevil5/upower-backlight-fix.diff?expand=1
Powerdevil fix for broken brightness
Patch applied, available in the build repo:
sudo pacman -U http://kaosx.tk/repo/build/powerdevil-5.25.0-2-x86_64.pkg.tar.zst