Yes, i can confirm this issue, which we had not noticed until now.
The problem is that each area uses internally its own spline to perform the edge clipping.
For surfaces, Forest extracts the edge of the geometry (in a process similar to Editable Poly->Border->Create Shape). For Paint, the area is converted internally to a spline.
But when a Paint area is used together a surface, Forest excludes the complete items which are outside of the surface... but the edge clipping is performed only with the Paint spline, not with the Surface spline.
To make it correctly, it should necessary to combine both the surface and paint splines, probably using a boolean intersection process.
I logged the issue in our to-do list, but i'm afraid it has not a simple solution. We'll see what can be done.
Meanwhile, i suggest you convert the Paint area to a spline (it can be done from the Area rollup). And then edit the spline, clipping it within the limits of the surface.
It can be done manually, or extracting the surface edge and combining both with the Editable Spline boolean tools.