I realize that this post is 5 months old but I saw it didn’t have a response so I thought I’d chime in.
Usually when they build the workouts they won’t make the actual timed interval be the kind that only advances with the lap press. Per your example, they will have a 6 minute interval that will end after the time runs out and then immediately follow it with a 0sec interval that will only end when the lap button is pressed. So if you are 10 min away from the hill that you want to do intervals on then you will end up with a 6 min interval (the pre programmed one) and then a 4 min interval (that ends with the lap press).
Hopefully that makes sense. Also, this could not be the case with the workout that you mentioned and they just made a mistake it creating it. If that’s the case I’ve usually sent a support ticket and they’ve been super quick with getting it fixed.