My A/C, when left on after I cut the the truck off and restart it, sometimes resumes in the defrost mode and then after moving the truck, it will return to the normal mode for Max Air. It will not return to the Max Air mode by just rotating the dial. Please advise.

Sounds like a small vacuum leak.

I tend to agree... A small vacuum leak will bleed of the vacuum and the doors will automatically reset to defrost mode. If the leak is enough then it might fight to return to Vents or other positions.

Check your other thread.