I also came across this problem.
To elaborate, I have a "grass base" object, which the forest uses to scatter grass. I have 1 spline defining the grass area, mainly to stop the grass from covering the entire grass base.
The spline only defines the outer edge, but for example I have a path through the middle of the grass, where there is the path geometry, but no grass base, the grass will overlap onto the path. This is presumably because there is no spline to define where the edge is?
Now I know I need a spline, even on the edge of a base object, but what would be very useful would be to make the edge boundary checking work when a spline isn't there, instead the edge of the base object defines the edge.
Does that make sense?