It's not clear where's your plane.
Have you changed/edit the profile for LSZH ? Have you seen the video ? It uses exactly C2, and the trucks are surely created behind the airplane. If this is not happening to you, please post a screenshot where your airplane can be seen there.
It's possible you have a real parking there as well ? In that case, GSX might consider the deicing to be like a on gate deicing, so it will call vehicles from afar.