i have the same problem.
and i'm sure it is an actual problem, a bug. just to be clear:
in a dark room:
if i set the backlight to go off after one minute of inactivity, it goes off after one minute.
if i set the backlight to go off after three minutes of inactivity, it goes off after three minutes.
if i set the backlight to go off after five minutes of inactivity, it goes off after four minutes.
if i set the backlight to "never" go off due to inactivity, it goes off after four minutes.
it will never stay on for more than four minutes, even if i set it for longer. except... if i set my "put the display to sleep when the computer is inactive" to "never" instead of my usual 20 minutes. then it works ok. and no, i don't have "automatically reduce the brightness of the display..." checked, and no, i don't have the screen brightness on the lowest setting, i have it on the highest.
why on earth do i want to do this? when i am watching a movie, or reading. i sometimes want to fast forward or whatever. so i want the keyboard backlight to stay on during the whole movie...