FINALLY!!! IT WORKS!!!
What works for me is this flag in /etc/default/grub:
nvidia.NVreg_RegistryDwords=EnableBrightnessControl=1
This is my grub file content:
GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nvidia.NVreg_RegistryDwords=EnableBrightnessControl=1"
GRUB_CMDLINE_LINUX=""
Then of course run: sudo update-grub
Source: https://bugs.launchpad.net/ubuntu/+source/nvidia-graphics-drivers-455/+bug/1905591
My eyes are saved :)