As this happens only with the iFly 747 and not other planes, it sound more like a calibration issue with your throttles. When the pushback is happening, it is over riding the forward momentum, when the push is finished then the thrust are pushing forward a bit until you hit the brakes.
Try the pushback without starting your engines and see if it does the same thing, or try starting the engines at the gate without the brakes on and see if it moves forward then.