The problem is caused by the scenery crashboxes, if the Marshaller is generated inside the crashbox of a building ( which might not exactly match the building profile, since the crashbox is a bounding box that covers the entire maximum width/length/height dimensions of the building, the ground there is no longer "on ground", but it's on the top of the crashbox so, when GSX creates the Marshaller "on ground", it will appears at the same height of the roof, because as far the sim is concerned, the ground there is that high.
That's one reason why we disable all crashboxes in our airports, which are also an unnecessary waste of performances (just to allow the sim to know if you crashed into a building) and they are a bit annoying too if you want to look the terminal interior with the Drone camera.
To fix this, you can edit the starting position of the Marshaller in the scenery customization profile, and make sure it's outside of the building crashbox. It might also be fixed by disabling crash with objects in the settings, but I really haven't tried this.
