It does sound like the rules are still active even after dropping it, but I know of no reason that an element that has blue squares for handles should move along with any vertex on any other element. (And it does behave like a separate element: When I click it, it says simply Line rather than Line String or Profile or anything else.)
I can drop the rules, but I'd rather not. I mean, what do I do next time this happens but on a bigger chain? And shouldn't tools behave as we expect them to? If we don't expect Drop Complex to work in this situation, what tool can we expect to do this?