The CSSTransform Pipe does not change the ExtendedPropertySet (GCoordSystemGUID) on the geometry field to reflect that the recordset has been transformed to another CSGUID. The GField::Get_CoordSystemGUID does return the correct GUID.
The CSSTransformPipe needs to have code added to handle the ExtendedPropertySet extension and its request for the CSGUID value (currently, no processing is occuring with the ExtendedPropertySet extension). The actual GField on the output recordset does handle the CSGUID correctly and is a faster way to get the CSGUID. It is also the recommended method.