Thanks, everybody. I was able to reproduce the symptom on a Lenovo Legion laptop in discrete-GPU-only mode: with a 545 driver, I can observe that the value in /sys/class/brightness/nvidia_0/brightness changes when I hit the brightness hotkeys, but the screen’s actual brightness remains unchanged. (I suspect it’s at the maximum level.) With 535, the physical brightness level changes as expected when manipulating the brightness controls. I filed NVIDIA bug #4364327 to track the issue internally.