It never seems to make any sense to me why, but increasing the "Shaded and draft quality HLR/HLV resolution" and "Wireframe and high quality HLR/HLV resloutions" within the Options>Document Properties>ImageQuality for the model itself to near maximums (usually within the red of the scale) affects the quality of the drawings output. If I understand correctly that your spline was generated in the part sketch, this might work. If it was created on the drawing sheet/file, maybe you would have to make the same adjustment to the drawing file instead.
Make sure to force a full rebuild in the model and then the drawing after updating the image quality variables (Ctrl-Q ).
I have seen lots of sloppy issues like this over many releases of SW. I hope your luck is better than mine in a somewhat easy fix.
You might also try printing to pdf instead of saving as pdf. That will often give better results.
I have had success with saving as a high resolution TIF file in the past.