Comment 1 for bug 168748

Originator: NO

That's because that line is not a path, it's a connector, as in
flowcharts. It has the inkscape:connector-type="polyline" attribute. So
when you move it, Inkscape optimizes the connector line as it normally does
in flowcharts and diagrams. You just need to have used Pen tool, not
Connector tool, for creating the line - that is not the right tool for your
purpose! And now you just need to delete that attribute using XML editor
and the problem will go away.

Still I agree that Inkscape should be more fool-proof here. It should not
optimize a connector line which is not connected to anything and was
manually edited in node editor. So I assign this to Michael.