XPS13-9360 - Brightness controls

Hi I'm having issues controlling the screen brightness on my XPS13, using the keyboard controls. If the laptop has been started up after being shutdown, all is well. The issue only appears after the machine has been put to sleep by closing the lid, or after inactivity. The control slider overlay appears and the bar goes up and down when the keys are pressed, but the screen brightness stays constant. This happens whether power is plugged in, or not. A reboot usually, temporarily, fixes the issue. Anyone have any ideas what's causing this, or can suggest a less radical fix than having to shutdown and restart? I'm on Win 10, build 1903, but have been having this issue for a while on older win 10 versions, so guessing that it's probably not an actual windows issue, but maybe more related to driver/hardware? Cheers Steve