In contrast this is what is happening with Xenial, which works perfectly.
evtest:
Event: time 1475641421.193234, type 1 (EV_KEY), code 225 (KEY_BRIGHTNESSUP), value 1
Event: time 1475641421.193234, -------------- SYN_REPORT ------------
Event: time 1475641421.193306, type 1 (EV_KEY), code 225 (KEY_BRIGHTNESSUP), value 0
Event: time 1475641421.193306, -------------- SYN_REPORT ------------
In contrast this is what is happening with Xenial, which works perfectly.
evtest:
Event: time 1475641421.193234, type 1 (EV_KEY), code 225 (KEY_BRIGHTNESSUP), value 1
Event: time 1475641421.193234, -------------- SYN_REPORT ------------
Event: time 1475641421.193306, type 1 (EV_KEY), code 225 (KEY_BRIGHTNESSUP), value 0
Event: time 1475641421.193306, -------------- SYN_REPORT ------------
udev monitoring in the attachment.