Hi Chris, do you know what program they are actually loading it into?

It could be possible that program does not know how to read spline curve entities from a DXF file, which is the kind of object that the 3DM to DXF converter puts into the DXF files that it generates.

If that's the case then you may need to use some other method to generate the DXF file that will convert it into a polyline. I've attached here the result of saving your DXF files from Rhino with the option enabled to save curves as dense polylines, that could work better for them.

- Michael