I think that, perhaps, the intention of Verisart is not quite applicable to the use case you supplied it with, and actually think that Verisart has behaved as expected in your case.

"Verisart applies blockchain technology to combine transparency, anonymity and security to protect your records of creation and ownership." (taken from the Verisart home page). I believe the important part here is the word creation.

We know about older pieces of artwork. We know where they are, who has them, who created them and when. What Verisart did, was certify that you have a reproduction of the Mona Lisa (in the form of a digital image). If you were to sell this reproduction, the buyer would see that it does not have the expected paper trail and know to pay accordingly for a reproduction (if at all).

I think where Verisart stands out is going forward. New artwork can be certified, on creation, by the artist. A potential buyer could see the profile of the seller and ensure it's who they expect it to be, or that it has the expected paper trail going back to the artist. If it doesn't, then it's a reproduction or at least highly suspicious.