It's precisely that the problem you're facing.
I've also tried to request GSX at EGPB (ORBX) and found that problem. The sloped afcad has no nodes, no taxiways, nothing. I've tried to add them with poor results. The vehicles used to drop or not show up. The flat afcad works just fine.
Maybe with this new version of GSX things have changed, but in all fairness, the issue lies with the afcad, and not with GSX. Maybe we can't have both worlds, i.e., sloped airports and working ground vehicles...