Actually, under the terms of CX bookable upgrades, it does say that J-F bookable upgrades are booked into A. So depending on how you interpret it (of course you may say it is subject to last seat availability but just clears into A for mileage credit purposes), the term itself it there. As a result, it would be somewhat reasonable to require POS A availability because the have to somehow rebook and reissue your ticket into A class.

It’s probably deliberate, but I have found many POS with very different F and A availability, even F6A0, when trying to plan my AONEx ticket...

https://www.cathaypacific.com/cx/en_...e-upgrade.html