This is definitely not correct. The first time I washed my car I opened the fuel filler door to dry the area. I then realized I had inadventantly reset my tank mpg display. But when I started the car I learned it was smart enough to know that I had not added any gas and did not reset itself even after opening the door.

I would go back to the dealership and insist they diagnose and repair the problem. This is obvious. If the computer is malfunctioning who knows what else is not operating properly.