Rear heat top vents

We bought this car in the summertime and now that it started cool off we are trying to use the heat. when turning the heat on in the rear once you hit 76° air completely stops coming out the top vents but it starts coming out the bottom vents in the back. is this by design or is this a part going bad? Hot and cool air works anywhere up front and in the back the cool air works up on the top that's it's only the hot air that won't come out the rear top vents.