This is tech support's reply to the problem.
Thanks for provided scene. First let me mention, that some time ago we've noticed an unwanted behaviour of Loft objects used as Exclude Area - for some reason the bounding box info required by our plug-in can't be accessed. Unfortunately, from our plug-in we don't have any influence over it. In this kind of cases, as a workaround, we suggest to apply the Edit Poly (Edit Mesh) modifier over such object to avoid this unwanted behaviour. Unfortunately, in this case it doesn't seem to work neither.
A "safe" way to get it working on any surface, would be to use a Spline Area(s) as depending of the surface topology, this spline is more or less adequate to be used as Spline Area. In some particular cases, it could result in some not ideally working shapes (even lead to self-intersections, that may create an endless loop in one of the Max functions which process it).
If similar problem occurs, it can be fixed manually:
1.- Create a Copy of the surface object & convert it to Editable Poly if needed.
3.- Under Polygon Selection mode select all faces used for scattering (sometimes the Select By Angle from Ribbon could be very handy).
4.- Having Ctrl key pressed switch to Edge Selection mode.
5.- Than hit under Edit Edges -> Create Shape. It creates a new Editable Spline object to be used with Forest.
6.- Select the Forest object, under Areas roll-out Add a new Spline Area and assign the shape created before.
7.- Select the Surface Area and turn it Off (Forest will not generate items from it).
By following these steps it'll give you nice and clean edges on any Surface(s)."
Attaching scene modified in this way (using two Spline Areas instead of the Surface Area). Hope it helps you a bit and please let us know that's all working fine or you are still facing some troubles with it. Thanks in advance and have a great day.