LED and some other displays are not always constantly on as you have found out. It is controlled by the microprocessor usually and saves power by strobing them. The rate of "on time" varies but, IIRC, there is a minimum amount they need to stay on else they'll get too dim. So the brighter, the more "on time" there is.
I'm posting this FYI. I used to design electronic equipment. Others would no more how to solve the problem.