What happened to my melted couverture?

I am assuming your couverture was real chocolate, since you haven't said.

While I don't know the effect of alchohol on chocolate, small quantities of water can easily seize chocolate. It becomes a nasty, pasty, stiff mess.

Typical 80 proof vodka would be 40% alcohol by volume, and so approximately 60% water, so your homemade extract would have had significant water in it.

According to Harold McGee as related in The Kitchn, this sounds like what happened:

The process of refining cocoa beans into chocolate gets rid of all the moisture, and so the final product is actually incredibly dry. Technically, even melted chocolate can be considered a 'dry' ingredient despite its liquid state.

For this reason, adding water to melted chocolate has the same effect as adding water to flour--it turns into a paste. Food science Harold McGee explains that "the small amount of water acts as a kind of glue, wetting the many millions of sugar and cocoa particles just enough to make patches of syrup that stick the particles together..."

You may be able to still use the chocolate for ganache by adding cream, or adding more water so it becomes smooth again, and use it as sauce... but it almost certainly cannot be recovered for covering confections any more.

To prevent this from happening in the future, don't get your couverture wet--from any source.

Note that the flavoring in truffles is normally in the ganache filling, not the couverture; you haven't said you are making truffles, but this gives you a hint:

One way to keep the couverture dry and still have your mint flavor is to add your mint flavoring to the filling.

Another would be to make your homemade mint extract with grain alcohol rather than vodka, so that there is no water in it.