Climate control not blowing through cabin vents 2017 S5 coupe b9

I have a strange problem 2017 facelift S5 coupe. AC works fine in the morning but if I leave the car parked in sunlight when the car is started climate control does not blow air through the cabin vents although the fan is running, checking under the bonnet below the black plastic covering at the base of the windscreen area I can see the recirc flap door is in the closed position. Cycling through all the different climate system settings makes no difference and the flap always remains closed unless I press the Max button at which point the flap will open and air will start to blow through the vents around the windscreen. I have found that in the car menu settings there is an option to tick a box for auto recirculation, however I have tried it with it unticked and ticked. Whilst Im driving the car the system will continue to not blow air unless I have selected max setting and when I park the car in my garage overnight the fault seems to reset and in the morning everything works correctly until the next time the car is left parked in the Sun. I suspect there is a faulty sensor in the system and have removed and cleaned with electrical cleaner the small brown sensor which is located just in front of the air intake under the Bonnet below the plastic panel as mentioned earlier. This has made no difference I assume there must also be a sensor inside the cabin probably within the rear view mirror housing however I have not removed the plastic cover has yet to check it. Previously my cabin filter was changed find my local garage while I waited the filter that was removed from the car was very dirty and looked like it had not ever been changed.
ideally I would take the car to the main dealer and ask them to investigate but I am worried that the fault will not present itself unless the conditions are right ie car left in the sun. Anyone with the same issue?