My family member had a recent issue… booked Avios redemption in J using QR Privilege Club for KUL-DOH-XXX.
During booking can only select seat for the ex DOH leg on QR metal, the MH codeshare doesn’t offer seat selection. Subsequently tried using the QR generated PNR on AY, RJ, UL, MH websites - all don’t offer seat selection. In the end all it took was a call to MH (quoting the same PNR) to book one of the throne seats on A330.