I am encountering a strange problem using a flight credit. I am trying to rebook using a flight credit as past payment. However, the "use flight credit" option does not appear!

After some testing I am suspecting the issue is that I am trying to book an international flight (Canada-US return). When trying to make a purely domestic dummy booking the "use flight credit" button appears just fine.

Have I missed something saying that I can only use flight credits on purely domestic bookings?