I paid for gsx pro and want to use it correctly. Fenix loads the data from simbrief and the issue happens just at 3rd party Airports. There has to be a solution for this.
But why you keep putting GSX in the picture ? It's completely unrelated to the actual change of Payload in them airplane, it won't touch it. I'll repeat it again, the one and only thing that would affect the airplane weight related to GSX, is when you refuel an airplane that has a default Fuel system, so it IS actually refueled by GSX, so its weight and balance will change.
That's the one and only case when using GSX would have any effect on the airplane balance. However, the Fenix is NOT refueled this way by GSX, so this is not the case either.
Having confirmed this is completely unrelated to GSX, since you said this happens only on 3rd party airports ( I guess it doesn't happen on *all* of them... ), I might provide you some hint from the point of view of an airport developer instead. It's POSSIBLE that airport is sloped, and some gates might not be entirely planar.
This usually doesn't happen in real life: it would be extremely dangerous parking (and especially loading) airplanes on a slope, so aprons are usually all at the same level, even if the rest of the airport might follow some slope. However, the MSFS terraforming system is not very predictable, and it's affected by many issues and even the terrain quality so *maybe* changing weight/balance on a gate with some slope might cause the ground model to put the airplane off balance.