you are slowly stripping the gear if you keep doing it.
the reason why its not doing it on the down side is it must be a little closer "as in the gears are closer" so it stops the motor before the gears slip.
So when your holding down it is burning the motor our not slipping th gears.
Alot of things do this, IE. power mirrors.
i dont know if this is normal. but i know how most of those thing work, and most of them work in the same way.
its not going to hurt it unless you keep holding it down. Just like if you keep holding you window up or down. your going to Burn the motor out