As general rule we don't provide backward compatibility. It may work or not, depending of the changes, but is not supported.
In this case, FP7 objects using Custom Edit mode are not compatible with FP6, because the items save format is different (include more values).
Unfortunately there is not workaround. To make it compatible, we should save the items in both formats (FP6 and FP7), duplicating the size in file. We have not plans to do it.
But usually new versions includes many other changes (i.e. new 3DS Max parameters), which may cause unexpected behaviours.
For this reason we don't test or support backward compatibility. Also this is considering only the immediately previous versions... but there are tons of FP releases, with multiple differences between them.
On the other hand, we just published Forest 7.0.8
, which should solve all issues commented in above posts. But if there is some problem yet, please tell us.