Ok, i think i have found the problem: Forest uses an internal cache to store the meshes. When the base object is converted to an editable mesh/poly, the cache is not updated properly, it holds a reference to the original mesh... which doesn't longer exists.
To fix it, you must reset Max and reload the scene. Once done this, the changes in the mesh should be updated properly in Forest.
Please, confirm us if the workaround works for you, to be sure this is the problem. We will fix this issue for the next patch.