I have a Samsung R70 laptop, and xorg just ignores when I press the FN key + BRIGHT UP (and down), while it works if I press that keys in a TTY. So as it is working in a tty (for example ctrl + alt + F1) and not working inside a running Xorg session (in my case a KDE one), leads me to think it is a xorg bug.
Xorg doesn't interpret brightness keystrokes on its own, this would be a KDE issue.
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