That's nothing that can be "fixed" easily, it's not obviously a bug, but it's related to the fact that the marshaller is not a real person that can easily alter his walking speed to match the airplane.
It's an animation that is made at a certain frame rate, so it would look funny and wrong if we altered its speed too much like a real person would do, so the min/max speed range the marshaller walks is more limited than what an human could do.
This means, he might be left a bit behind or being too close, depending how the pushback path it's done, its length, etc,. so, it would be very difficult to match the correct distance, without having the animation looks unrealistic.
