I have the same problem and it definitely happens when transitioning through the different stages of my booting process or unlocking process.
I can turn it off each stage and then the next stage it will come back on.
- Boot to grub (backlight comes on) and I unlock my partition for booting into linux. I turn off the back-light with the F10 key (Dell XPS 13)
- Goes into LightDM Greeter and the back-light turns on. I then have to turn off again with the F10 hot key
- Logged into mate desktop and back-light turns on again. I then have to turn it off again with the F10 hot key
This looks to have been resolved in UPower (but I don’t think mate uses that) as the fix didn’t work for me.
It would be great if we can get someone to help us troubleshoot this type of problem.
The solution that would be highly recommended for me is the one that works in Mint which is that the ambient light sensor is used to tell the back-light what level to be at (known to work on Thinkpad and Dell). But I know that is wishful thinking on my part.