New grass system

When I use a spline on my landscape to create a river (incl. deforming the landscape underneath) the new landscape grass system puts grass over the spline (river) as if it wasn’t there. Do I need to use a different layer or maybe paint grass manually ?

Ok I changed it so it’s now using a layer without grass (dirt) but still grass is appearing above river. Almost as if the new grass system doesn’t take spline (deformation) into account.