Apparently when a ticket is reissued, the airline involved may issue you a MCO for the taxes that is being refunded but you may only claim the taxes from the original issuing airline/office.

CX once reissued a RTW ticket for me, and issued a MCO with a difference to return about $100 in taxes but they advised me that I can only get the money back from AA in CAI (then) as they were the ones who issued the original ticket.