I think that the restoration ceases to be the work of the original artist and start to be the work of the restorers when first, the art they are restoring loses the original intent of the original artist. Also, I think that when the form of the art and visual techniques are altered in some kind of way, then it becomes the work of the restorers. .
When an artist makes a work of art, they have a purpose for the viewer to feel. They want to give you some sort of feeling toward the art, and when the restorers change the art until the point where the original impression is gone, then the work ceases to be the work of the original artist. It is true that for each person who sees the painting the impression you get is varied, but the original impression that the artist tries to leave on you is the whole reason why he drew or created the art in the first place. Art comes from the heart of an artist or sculptor, and to change that means that you have changed the whole art, and then it becomes the work of the people restoring it. .
The thing that distinguishes an artist's art from the others is his or her technique and their own unique visual style. When a restoration process gets rid of that by restoring it so much as it becomes non-visible or if they restore it so the whole art looks different, then it ceases to be the work of the original artist. If a piece of work becomes so different as it doesn't look like the original, then of course you cannot recognize it of being the original artist's, because it deviates from their particular style. If you cannot recognize the particular visual effects the artist had from the original work, the art takes on a whole another new meaning, and naturally leaves a different feel on you. It may give you less of an effect and influence from the original artist, because altering the visual effects changes how an art looks like. Because of the fact that small, little details help distinguishes the perception of art, altering that just a little bit has a huge effect.