Sorry if that title is confusing, here's an illustration which makes it a lot clearer:
What I do:
What I hoped for:
What actually happens:
The tricky thing is it seems the belt segment which is ultimately deleted is first replaced with the "up" underground segment as soon as I drag across it, and then that gets moved to the final position as I continue dragging left. So the perfect result would definitely require some extra logic. Perhaps that's getting too complex, but none the less the current behavior is awkward and comes up with some regularity while doing main bus designs.