The reason why that happens because during DWG export ArchiCAD converts symbol fills into lines. To avoid this you can set fill/hatch conversion match table in DWG/DXF translator. Fill conversion match table available only if you are using DWG file as template. You need to create DWG file, where all needed AutoCAD hatches are drawn (let say as squares). DWG translator reads from file information about hatches and able to choose a hatch to replace ArchiCAD Fill.