First let me mention, that's most probably not related to the Forest Pack plug-in at all (please feel free to do a quick test using Forest Tools
and export all distributed geometry to instances - this way you can easily check, if the problem is related to Forest object(s) or not).
As far as I know, PhysX (MassFX) might have issues with groups being used in scenes and Ungroup
these can help. Anyway, I would encourage you to contact directly Autodesk
regarding this problem as I'm afraid we can't do much more in this case. Thanks for your comprehension.