Design Workshop Lite: Export 3D>DXF Model then Import>DXF

[Brian Horton]

If I create a simple block with a window through it then
Export 3D>DXF Model
Then
Import>DXF Drawing or Model
From the exported .DXF file, the result doesn't quite work:
1) Lines have appeared between the corners of the window and the nearest corner of the block
2) The window doesn't go right through
3) The polygon faces of the block and window are not grouped, though apart from issue 2) above, it does look almost correct in View>Lights & Textures
Is this a bug in DWL or a limitation of the .DXF 3D format or am I doing something wrong?
Brian