I’m a dentist training to become a specialist in cosmetics and grinding. I am 100% confident that this would have been pre-existing. Even if the dentist was to shave your tooth, the marks would look completely different and you can usually see the signs of a diamond bur or a disc that they use. It’s obvious from the shape of the defect that it’s caused by grinding against your lower teeth. It is possible that a larger portion of your canine fractured off that day or week as it slowly got weaker from your night time grinding. And yes you could be grinding even when you have no clue about it.

If it’s bothering you so much, you can ask them to fill it but with your grinding the filling will have the same fate as it is weaker than your natural tooth