Just comes down to yield management. All airlines actively review the availability of FF seats, and availability can happen at any time before departure. It is, after all, one of the reasons why waitlists clear.
There is a careful balance between rewarding FFers (long terms availability) and wanting to discourage otherwise full fare paying passengers from snagging award seats instead of paying. But in the other hand seats in higher cabins can become available to ‘reward’ those seeking upgrades, or even just those lucky enough to come across seats like you have.
CX is well known for releasing awards any time from two weeks before departure, sometimes the entire F cabin bar one seat. Other airlines too, like TG and LH (famously with F at T-14). But of course no guarantees (to encourage those who are going to pay, to actually spend their money!)
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