The section you took through your 3D model resulted in various curves and polycurves - they are not polylines. When you export them, they are indeed turned into polylines due to your settings here:
If I export your curves with the same settings you have posted above, and then re-import them into Rhino, I get a very close approximation of your original - of course, all the smooth curves have been converted into segmented polylines, as per the export settings.
So the fault seems to be in AutoCAD’s import perhaps. I don’t know what version of ACAD you are using, but since you are exporting from a LEFT viewport in Rhino, this can confuse versions like LT which are strictly 2D.
I wonder if you can try the following:
Use RemapCPlane on your section curves, remap them from Left to Top. Then project the result to the Top CPlane. Then re-export the result. Does that work any better?
The main issue I have is that this used to work just fine and no longer does. I wish I could say when this started happening - from memory it was before version 7, but I can’t swear to that.
I don’t think that the viewport selection makes any difference, since the setting forces the curves to be projected onto the CPlane, and are exported as a 2D drawing. We’ve been doing this for a decade+, and that was never an issue.
And on my end, if I re-import the curves into Rhino, I get exactly the same result as in Autocad, which tells me this isn’t an Autocad issue, as much as that would have been satisfying…
Rhino is doing something to the curves on the way out.
Which looks correct to me and there doesn’t appear to be anything missing.
Interesting though, you are showing it in the XY plane and somewhere near the origin (you can see the grid and a point in your image) but the curves were not on the XY plane when exported, they were drawn in the “Left” ortho viewport.
Exporting Polylines as Splines turned out to be the most appropriate way to all the CNC laser shops that used my DWG and DXF drawings over the years. Some of their software was unable to properly work with Polylines saved as Polylines, however, Splines worked 100% of the time.