Very interesting indeed, you nailed it. The thread "CX-865 flight availability when only going to YVR" in the Cathay forum suggests that there is an annual or monthly quota/cap on tickets sold on this particular fifth freedom route (JFK-YVR). In fact, it appears that Cathay is selling no tickets--award or even paid full-fare(!)--through the end of February. This explains why the award availability suddenly went to zero, despite the seat map showing a half-empty plane. This is definitely something to consider when booking this route, especially since I intended to take advantage of the award re-deposit EP benefit to tailor the dates of my trip close to departure. Luckily this time around, my original travel dates will work. But be prepared for little to no last minute flexibility on changing your departure/return if you book onto Cathay 865.