This is a vacuum thing. The system defaults to your condition when there is no vacuum. You probably have a vacuum element leaking. The A/C position requires the defrost doors to be held closed, the center vent held open, and at least one stage of the recirc door pulled. Any leak from these posible elements causes all of them to relax to the default position. This position is defrost for safety reasons; thats the best thing to be stuck on.
Bosch Master, ASE Master, L1
33 years MB technician