There’s no mention of your environment - but with the display manager being SDDM I am guessing KDE Plasma? Am I right?
Sometimes changes within KDE can cause malfunction if one is using custom theming.
Suggestion of items to check
- configurations are stored in /etc/X11/xorg.conf.d
- verify you do not have any configs stored in /etc/X11
- verify any configuration in /etc/X11/xorg.conf.d does contain duplicate settings
- try removing acpi_backlight=vendor from /etc/default/grub then rebuild grub config and restart