It's possible the motor that controls the ducts/vents is flaky. That or some of the wiring. That's one of the drawbacks of these systems. Could also be the vent controller too..depends on how its all wired in.
Anyway I have a similar problem, where my vents are all on at the same time in the front no matter what setting I put it on. Kind of irritating, but should be a quick fix ... hopefully not expensive lol
Gonna track down a factory service manual so I can break this out | 
