Certain laptops with a mobile Nvidia GPU require the nvidiabl driver to be able to adjust the screen brightness. This driver uses the /sys/class/backlight/nvidia_backlight interface which is not recognized by power manager. Although I haven't tested it out myself yet, I believe that adding nvidia_backlight to the list of backlight interfaces in root/src/xfpm-backlight-helper.c will fix the issue. For proper priority, it should be placed above somewhere above acpi_video0 in the list. If interested, the nvidiabl source can be found here: https://github.com/guillaumezin/nvidiabl
If you can test your proposed solution, we would add the backlight to the list. Please reopen if you feel like it.