Comment 3 for bug 492318

The line connecting A and B doesn't have a 'middle' node therefore no mid-marker is rendered. I don't how you created the other (split) connector line. The path isn't connecting B and C directly and behaves unexpectedly when moving B or C.

See also related report about the ui for markers:
bug #168670 “Not obvious that mid markers only draw at nodes”:

and connector tool related:
bug #168748 “Connectors with manual node edits should retain their shape”
Bug #167210 “3-point line loses middle point when moved”
Bug #171933 “curved connectors”