I have been in projects where this have happened a couple of times. Not my mixes but when supporting a good friend and producer as an adviser, it happened twice. One pure theft like what happened to you, and once where he was offered a partnership and asked to write and sing the vocals of an instrumental progressive EDM track. Fortunately he came to me for the ”judgement call” on the result, and I recognized the track as one of Maison & Dragen. It was a great vocal, but we reported the faker and got away before being dragged in to shame.
While being flattered and angry at the same time, hang him out to dry. Put links to the scammer and tell everywhere what a fake he is, while also linking to your original mix. That way you can pick up some followers that he might have gotten on your behalf.
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