I'm using Asus Zenbook Pro Duo, Intel Core i7 and Nvidia RTX 2060.
$ ls /sys/class/backlight intel_backlight
When I change the brightness from the keyboard, it shows values going up and down, but it doesn't actually reflect in the brightness levels. It always stays full bright.
The following values change from keyboard interaction, but don't change screen brightness.
$ cat /sys/class/backlight/intel_backlight/brightness 14496
$ inxi -G Graphics: Device-1: Intel UHD Graphics 630 driver: i915 v: kernel Device-2: NVIDIA TU106M [GeForce RTX 2060 Mobile] driver: nvidia v: 455.38 Device-3: IMC Networks USB2.0 HD IR UVC WebCam type: USB driver: uvcvideo Display: x11 server: X.org 1.20.9 driver: modesetting,nvidia resolution: <xdpyinfo missing> OpenGL: renderer: Mesa Intel UHD Graphics 630 (CFL GT2) v: 4.6 Mesa 20.2.1