I had the same bug as you and I have just resolved it following these steps:
Creating the file /usr/share/X11/xorg.conf.d/80-backlight.conf and filling it with:
Identifier "Intel Graphics"
Option "AccelMethod" "sna"
Option "Backlight" "intel_backlight" # use your backlight that works here
Bassically, I have followed the steps described in the section "For the Intel HD Graphic Controllers" from https://wiki.ubuntu.com/Kernel/Debugging/Backlight (this link was reported by you, thanks ;) )