I can't see why this can be a GSX problem: GSX just reads the actual payload from FSX using Simconnect, and it does it in the same way, regardless of the aircraft used, and it's not affected by the aircraft customization, it's entirely standard method, and since you said yourself it works with the A320, there's no reason why it won't with the A318, the only possible explanation is that the airplane WAS empty, for some reason, at least this is what the airplane reported to FSX.
And of course, just to be sure, I tried it both with the A318 and the A319, and there are no problems whatsoever, I can see the normal baggage train with 3 wagons, and the large passenger bus (which of course won't appear if there's a jetway), meaning GSX has got a normal payload figure from FSX, as it always did. No difference if I use or not the supplied fuel/load planner, it works in both cases.