Why foreign flag rank did not carry back into Royal Navy service

Would a Royal Navy captain who became an admiral in a foreign navy return with that higher rank recognized?

No. He would still be a Royal Navy captain, although he might be appointed commodore for a particular command.

A half-pay captain might become an admiral while organizing another country's fleet, but that foreign promotion did not alter his place in the Royal Navy. On return he would still be a captain, though the service could employ him as a commodore for a squadron or special duty. Commodore was historically an appointment signalled by a broad pennant rather than a permanent Royal Navy rank. An officer could therefore command as a commodore and still retire with the substantive rank of captain.