note-anchored line should connect notehead centers
a note-anchored line (Add > Lines > Note-anchored line), apparently connects the left edged of note heads, ist should connect their centers. So while being vertically centered it should also be horizontaly centered. A horizontal offset of 0.5sp should take care of that (not perfectly though, as notheads, while being 1sp hight, usually are wider than 1sp).
Workaround of course is to manually apply that offset