Thanks for the tip Inf1n1t.
What’s puzzling is the way it renders a line on top of the object is not consistent. In fact, if you look at the two smaller spheres in the my previous screenshot, those lines are actually “in front” of the object, yet they are rendered the way I’d expect it, by mixing the two opacities.
It just seems that for some camera angles that opacity mixing is misbehaving, causing some line segments to be completely solid.
I’ve tried the blending settings you suggested, both on the lines and/or sphere objects. No combination of blending makes any difference except when selecting NoBlending (effectively cancelling the object’s transparency) or SubtractiveBlending which makes matters worse by always drawing dark lines on top of the spheres:
