I’d contact BA in the first instance and ask for confirmation your exit data was processed properly.
Secondly on the next time you visit the US take proof with you in documents you can leave with CBP. A print of your boarding pass, a print of the Avios for that flight in your BAEC account, a card transacti9n slip outside the USA, a boarding pass of another flight outside the USA.
You will find that it may take a few minutes more to be processed but nothing too onerous.