"Thats good, but we lost all controll of scale and density of the bushes near the edges."
My apologies, I didn't notice, that newly created "Shape002" Spline Area covers also the area of "Shape001". The Density / Scale Falloff is applied, but for both of these Spline Areas and it's overlapping in this case. That's easy to fix (I've send you modified scene) by attaching copy of "Shape001" to the "Shape002" Spline Area.
"If i understand ur explanation correct. The problem appears because there are 2 geometries set as surface. But if i delete the Google Earth Terrain_002. And only have bushes on one surface, the disappearing issue is still there!"
Not necessarily that might happen just with multiple overlapping surfaces. As commented previously, it's depending of the surface topology. The "virtual" spline is more or less adequate to be used as Spline Area, but in some cases may lead to not ideally working shapes (as you've pointed out, in your case it's probably linked to the "Büsche Source Collapsed" surface itself) thought it seemed to be a "stacked surfaces" related.