Red Green wrote:

Forget the flip and polish. They should have been smart enough to hide it
up front. If they object, ask how it got through inspections.

It's a cosmetic blemish, so there is no inspection other than the customer.
Looks to me like they screwed up with the saw and tried to use the piece anyway.
Even if it doesn't bother you, it may bother a potential buyer, so I'd make them
replace it.

Fortunately, the material is relatively cheap and because it's a backsplash and
not a surface joint, it doesn't need to match exactly. It's also mechanically
easy to replace, so the installer isn't going to be out a lot of time or money
doing the job the way it should have been.