PROBLEM
Once you export B-spline surface as a DWG/DXF and try to import it again, the surface does not look the same anymore – some coordinates are shifted, and the surface does not perfectly match the one before export.
SOLUTION
In Save As DWG/DXF Options > Entity Mapping > ‘Flat Solids and Surfaces’ and ‘Curved Solids and Surfaces’ change values to ‘Polyface Mesh’. The surface now should look the same as before export, but also it becomes a polyface mesh.