Is our old friend "married segment" raising its ugly head again.
In short flights connecting A to C via B may allowed but not A-B or B-C.
Not uncommon on Qatar or Cathay.
Qatar is known or selectively releasing award seats to airlines ( particularly mean to QF) so what is offered to, and available on BA or JAL or AA websites is not what is necessarily available to QFF fliers. This happens regularly on Iberia and in particular Air Nostrum . Far more business class availability on BA website vs QFF one ( maybe that's because BA and Iberia are owned by the same company - International Airways Group )
Re "phantom availability " there are some rational explanations for this .
One I know of is this
An airline say BA has a Qatar award seat available on their FF site - it is available on. QFF .
The seat is taken by someone out there in the frequent flier universe but individual airlines request or are given award availability at different times eg 4 or 6 hourly dumps .
So I can understand that at one particular time ,
the QR seat is no longer available but BA may still be offering it and it no longer exists on QFF.
Of course , this is just guesswork on my behalf!