Dragging the ends of an 8va line with the mouse shows a purple line indicating what note the beginning or end is attached to. When you drag (or shift arrow) the line longer or shorter, the purple line doesn’t move with it. This results in incorrect playback. The notes you dragged the octave line over sound in the octave before the drag, not what the octave line is telling it to do. Seems the purple indicator should follow as you drag the 8va line from note to note. Even if you cut and paste the section, the purple line is attached to the original note, not to where it was dragged.