Backlight control won't work after suspending and resuming
Since updating my system (Solus 4, Xorg) to 3.36.0, it looks like the backlight is not adjustable anymore after closing the laptop lid and re-opening it in some cases. Suspending and resuming again fixes the issue.
The device is a Dell XPS 9370, and the issue may even be related to connecting ("hotplugging") external monitors via the DP/Type-C interface.