When lengthen or shorten a note under an 8va line the first note is displayed in it’s original octave.The next note will be ok.
This is not a bug, as such: the length of the octave line is not tied directly to the duration of the notes underneath it, rather to the initial duration of the notes at the time it was created. In the forthcoming 1.1 update it will be easy to resolve this simply by grabbing the right-hand end of the octave line and dragging it in Write mode.