If you see abrasions on the diamond plate where it was in contact with the truck bed, I would agree with the dealer that you pulled it off when offloading. If there is no sign the the diamond plate is scuffed, it may have been poor application of adhesive from the factory. My AF811 has a couple of surface scratches where it came too close to my tailgate opening, but I have had no separations or dents in the diamond plate.
To fix this properly would require removing the diamond plate and old adhesive before cleaning and applying new adhesive.