With 381.09 and backlight control finally working with Pascal-based cards, I’ve found a bug when waking up the screen.
If the screen is turned off via DPMS (i.e., not just blanked via an image file or the like), a subsequent wakeup will reset the backlight value to full brightness.
The previous value is “respected” in the way that it’s still set correctly (for instance, xbacklight -get and the sysfs node report the old value.) Changing the value by a step up or down pulls the brightness back to around the level (plus/minus the stepping value) before DPMS kicked in.
It looks like the driver is not correctly resetting the brightness value after the screen wakes up.