Airlines started incorporating USD $10 DR tourism tax into ticket fares starting in 2018 (it used to be payable on arrival at airport). You can see it when you do a tax breakdown of ticket price. Perhaps AA is not refunding this tax for some reason (maybe related to the fact that it didn't used to be incorporated in tickets). Currency conversion changes shouldn't come into play on US purchased fares as I believe all DR taxes and fees are charged in US dollars.