Belgian DPA prohibits the transfer of tax data of Belgian Americans to the USA

"Article 96 cannot be intended to allow that international agreements remain contrary to the GDPR over time. The exception provided for international agreements concluded before the implementation of the GDPR does not exempt EU member states from (re)negotiating an agreement to make it GDPR compliant."

While I know nothing of FATCA, the conclusion here seems quite substantial as there are supposedly plenty of international agreements that are potentially affected. It is also interesting that Art. 96 hasn't come up earlier even though its message is very clear.