The problem is with the sending unit in the gas tank or that circuit. It isn't monitored by the Engine Computer so it's not going to set any fault codes, although on a car that new, and with all the insane, unnecessary electronics, there may be a Body Computer involved, and on Fords, the most complicated computer on the car is the instrument cluster. Either of those could cause that symptom too. That also has nothing to do with engine performance so the engine can run fine.
Your mechanic can connect a scanner to see exactly what the sending unit is saying, and he should be able to operate the fuel gauge remotely to test its operation. You can also unplug the sending unit, then ground the wire to see what the gauge does. One condition should make it read "full", and the other condition should make it read "empty". The problem is when they add all the silly computer circuits, there are a number of variables that can be added. Examples of those could be that the gauge reading will not change under any conditions unless the engine is restarted or the car is moving. Sometimes they incorporate things like that to prevent varying readings when you go around corners or over bumpy roads with a low fuel level. That can make diagnosing the system rather difficult.