funny i was just having some issues with game exporter and found this thread on first page from google search, so i'll update with solution to a similar problem but different reason: using export selection with the skeleton reference group selected works properly, but using the game exporter with same selection does not.…
Next thing I would do is duplicate the problem mesh, delete history, do a humanIK quick bind and then export that and test it in engine. If that works correctly, then you know the problem has to do with the mesh itself rather than the import/export process. If this is true, then I'd probably start by copy the current skin…