We used some hatch patterns in our display model which we loaded via StyleEditor from external files (xml, pat). We also made sure that corresponding AutoCAD hatches with the same name were available when conducting DWG export - but theses were always missing in the drawing file.
It turned out that the following tag in the layer is responsible:
It automatically gets created when you load a hatch from an external file. After removing the tag DWG export worked as expected. Same happens by the way with lines styles.
Map 2103, SP2