I've noticed it with section curves of drilled surfaces like spirals or lofts: Though a curve looks like a straight line, it can't be selected as such a line. I've tested it with AutoCAD, Rhino and Inventor. In AutoCAD and Rhino a 3 degree curve is noticed, in Inventor nothing is known about the curve. A workaround would be a re-draw of the curve, but ..
Fortunately, in Rhino the degree of curve can be changed to 1st degree, resulting in a straight line. Alias can do similar, but I didn't test it. See description: