I have a solution as well. I went to Dell Support and installed NVIDIA GeForce RTX 3050 Ti/3060/3070 Ti/3080 Ti Graphics Driver from the Drivers Tab. Hope this helps
Roughly two weeks ago, I started experiencing problems with my brightness level. I keep adjusting the slider from the keyboard and windows settings, but it does not decrease the brightness level - it is stuck at the max. A simple restart would fix this in the beginning, but not anymore. Yesterday, I tried reinstalling the AMD display card driver and the Nvidia one, which seemed to fix it until the next shutdown. Today, when I started the device the slider, does not work again.  Has somebody experienced the same issues and if yes, please how did you fix it? 
m15 R5, cannot adjust my brightness level