Currently when lengthening slurs, the code only tries to do that only in the same voice, you can’t lengthen slurs beyond the end of a voice. In the attached file, the downstem voice in the first bar is a different voice to the downstem voice in the second bar (you can see that if you turn on voice colouring in the View / Note and Rest colours menu). At the moment, you have to reenter the slur to end in the second bar.
We will think about how to improve this behaviour.