Hi,
Thanks for reporting this issue. We're able to reproduce this problem of referenced Forest object as a XRef Object using the mental ray renderer (other render engines doesn't seem to suffer this kind of troubles). Our main developers will have a look at it, but let me mention, that we're now very busy working on the upcoming RC3. The XRef Scene would be currently the recommendable option. Thanks for your comprehension.
Again, big thanks for your feedback.
Best regadrs,