When you export DWGs in Revit, filled regions are not exported correctly if some filled regions overlapped each other on the views. Here is a example.
In Revit 2017, there are floors which patterns were applied, and also another filled regions on the top of those floors.
If I export a DWG file from this view, some patterns will not be exported correctly. This is what it looks in AutoCAD.
To fix this issue, you should change the Visual Style of the view from Hidden Line to Wireframe.
Once you change the Visual Style and re-export the view, then you will get a DWG file which has all patterns are exported correctly.