This is not a bug, and GSX has always been like that since the first version. The baggage train for a cargo door that doesn't exist (in this case it's the Main Cargo door) was always placed on the parking, but it would go away because it's not needed. The idea is that train was already there in the parking, but it went away if it wasn't needed for your airplane.
Of course, you might have not noticed it before, because most of the GSX airplane configurations are for two cargo doors so, when GSX only supported two loaders, it was uncommon to see an "unused" baggage train to go away, unless you had a plane configured with a single door.
Now that we support 3 cargo doors, it will be more common to see the 3rd unused one to go away, so you see something new (which was always like that), and are assuming there's a problem, but that's how GSX always worked.
Maybe we might change this behavior by having that vehicle appearing only at Cargo parkings, this would make it more realistic, since we might assume it was there because the previous plane that went there *had* a Main Cargo door.