I have a 2016 Mustang GT with the same shifter.
To put the car in reverse, you have to lift up on the metal ring below the shift knob in order to get to the far left reverse location. Everyone, and I mean everyone, puts the car in 1st gear the first time you drive it and think it is in reverse because they don't know about lifting the metal ring. If you don't see the back up camera image on the touch screen, you are not in reverse, you are in first gear.
This was a simple mistake that is easily made, and was made by myself the first several times I drove my car.