Ultimately you have to rely on what you can see on qantas.com.Does anyone have an explanation?
seats.aero can help you find dates to check, but it could be displaying wrong availability for any number of reasons (eg out of date results, bugs, etc). Whatever you find on seats.aero, you should go check on qantas.com. If it's not on qantas.com, you are wasting your time wondering where the seats are.
It is extremely common for airlines not to release all their award seats to partner airlines. Indeed, Qantas is notorious for doing this to Alaska and American, so it is only fair that they retaliate in kind.
