any FSDT airport, has vehicles, of it's own, and these also leave spawns.
They don't. P3D does it.
If you can think of a possible fix
There's no possible fix, it's not a bug of the scenery, it's something that P3D does on its own and FSX doesn't.
Those are not just traffic, every 3d object in all our sceneries is created programmatically using Simconnect, and there's no other way to create an object other than the one we use, which is of course the one and only official sanctioned way through the SDK.
FSX doen't exhibit this behavior, so it's clearly a P3D specific problem, because it's to be expected that both would work the same, using the same (legal) Simconnect code that works correct AND per SDK specification, under FSX.
It's not the only difference in behavior between FSX and P3D. The transparent ATC menus also works differently, they stop to reply to key press if the menu is not in focus, while FSX the key continue to be processed as long the menu is displayed, even if the user clicked on a different window. This affects GSX and any other add-on that uses menus (including the default ATC menus), and it's also due to a P3D difference we can't do anything about it, it has to be fixed by LM.
We reported the menu problem more than 1 year ago, but it hasn't been fixed.