Even with paid tickets, this is one of the wonders of yield/award management. When a single seat offered in a given fare bucket is booked, the computer software will decide whether to release another single seat for sale to that same bucket. It will revalue the whole booking situation of the flight (paid and award) . I guess that it could be done in batch nightly, or instantly.

I am not saying that this is necessarily what happened. But I would suspect that the award management system has all kinds of computerized rules. I have been frequently searching for 2 awards on CX and CX often only release one award space at a time. But when I book that award, I sometimes found that a new one was available the next day.