Hello,
Believe it or not this something really stupid! I used to have the same problem with my Lincoln Town Car! This issue is cuased by the CLIMATE CONTROLLED SELECTOR SELENOID, it seizes in the middle when you shut off the car & then when you turn on the car it resets which it pops back into place and then works perfectly! Why it does this has a few explanations:
1: when vehicle was assembled it wasnt leveled
2: the person that mounted the selector box did it half ass!
3: defective batch
This will not cause any serious issues that will cause death or fires or even a paper cut! All that will happen is it will either wear itself out and work correctly or wont work corectly! when it goes bad, It will always blow threw the vents just not vents you want! Ask your service manager to pls look into this and you and him will probably be surprised!
Im young but have been in a mechanic shop since the age of 6months as my father owned his own and i also have 3 shops of my own at this time! I specialize in electrical work and acc, but also have 4 mechanic bays in each shop! GOODLUCK!