This reminds me of something I did in my audition for my apprenticeship.. And haven't done since. The ghost line of where the grow out is is at least a shade darker than the ends. It doesn't look like they were going for ombré ...so to my mind it just looks like a mistake. Or, Junior styling. I know that sounds harsh but I am really just pointing it out to educate. A master level colorist doesn't foil hair and have a ghost line of grow out.
Here is an example of work I did on a heavily grown out client, compounded further by the grow out being from solid all-over color:
Not to blow my own horn but this really was one of the more challenging colors I've had this month. To achieve the finished result, I did micro slices on the diagonal of highlights and lowlights.
This kind of work is difficult but so very rewarding when my client leaves with glorious, natural looking hair and there's no ghost line advertising she wasn't just born that blonde ;-)