Dear Samppa,
your observation is right, this time the Floor Plan view of Shells cannot be exported to DWG.
It is a known issue which is scheduled to be fixed in the next Hotfix of ArchiCAD 15.
As an other workaround I can recommend to explode the Shell with keeping original elements too (Edit/Reshape/Explode into Current view). This option will turn selected elements into basic elements which can be correctly exported to dwg.
Regards,
Katalin Takacs