2015 jeep cherokee check engine light is on, heavy smell of burt plastic

Bought a brand new 2015 Jeep Cherokee Latitude with a full tank of gasoline and 6 day later, at 102 miles, the check engine warning light came on. Also with it, came a strong odor of burnt plastic or something and the transmission was kind of shifting rough. Took the vehicle to the dealer and was told, a day later, that the cause of the problem was the fuel cap loose and the odor was normal for a new car. Since I bought the jeep,The fuel cap has never been mess with.  Is this normal for a new vehicle to do the above mentioned situation?