Comment 9 for bug 1510344

I was facing to this behaviour.
I blacklisted the dell_laptop module and this issue was solved. Now my keyboard backlight act as it should be.

in /etc/modprobe.d directory
create this file blacklist-dell.conf
and add in it the followinf thing : blacklist dell_laptop
then excute : sudo update-initramfs -u
and Reboot