Had the same issue with the same flight recently. We'd rolled the dice with a certain OTA we booked through and they hadn't ticketed the booking, and after a few days it expired. A phone call later and it was ticketed, and we phoned BA who were able to verify this (only the first flight was BA ticketed, the rest were QR flight numbers). Once it had been ticketed we were able to book the seats on Malaysian website which then showed up on BA and QR, although still not able to access MMB on QR website.