Well, though Forest objects can be nested, instancing is applied only on "the top-level". Any "inner objects" are converted to mesh. In the case of having "the inner object" with many polygons (like in your case), the Display > Max. Faces limit (5 mills. face by default) might be reached. Raising such a limit for the "FP_PATCH" object or lowering the diameter of the assigned Spline Area "Circle001" will fix it. Let me add, that depending on the render engine (and its release), the Render Mode for used "nested object(s)" might have to be changed from Automatic to Meshes under the Display roll-out. I hope that helps.