I think you'll find that's the expected behaviour. On some older model cars, you will find the icon on the dial for face/feet has a blue arrow for face and red arrow for feet. I think it's because engineers found it was more comfortable to have cooler air on your face than the hotter air on your feet.
Try doing a HVAC door relearn.
Ign on but engine off, press and hold the rear demister button till you hear all the actuators moving.
They are fairly quiet so close all the doors and windows so you can hear them working.
Edited: sorry, I quick answered without reading your entire post. You already did this :bop:.
Being well within warranty, looks like just take it back to dealer mate. Maybe one of the doors is actually stuffed.