Fully updated but vanilla MATE install, brightness change hotkeys aren't working on my T530 running Optimus. I've noticed this issue on most other distros up until about a few months ago. FWIW everything works flawlessly under Budgie, and under current builds of Gnome and KDE on pretty much any distro. The brightness OSD changes, my guess is it's writing to the wrong backlight interface in sysfs (that's what I've seen before).
Let me know if I can track down any other info, I'm not familiar enough with all the moving parts in here to know where to start finding actual hard data.