It's a known issue, shadowvolumes were tagged in a way that none of the importers can recognise.
Use the lowrez as a basis for the shadowvolume. There is also a trick to doing them for interiors. The faces have to be inverted for the shadows to be cast correctly.
Use the lowrez as a basis for the shadowvolume. There is also a trick to doing them for interiors. The faces have to be inverted for the shadows to be cast correctly.