This isn't necessarily a 6/6+ issue as the same thing happened on my 5. When I try to scrub through music using the music controls either on the lockscreen or the control center, the scrubbing is never smooth. It is always erratic and jumps all over the place. It's nearly impossible to skip ahead or behind a few seconds, as it will just jump around everywhere. Anyone else experience this? Is there a way to make it better?