I’m having the same issue tbeckman reported on an Alienware 15 R3 with nvidia driver 465.24.02, while everything works fine using 460.67.
I’d like to add that in /sys/class/backlight/nvidia_0 folder, the values within brightness and actual_brightness do change when using hotkeys or tools like xbacklight, but it looks like they are not honored anymore.
I’m attaching a bug report hoping it may prove helpful: nvidia-bug-report.log.gz (396.8 KB)