I've seen similar. I've been looking for either WT+ or Club having originally booked into economy for 3 adults. This week a lot of Club and WT+ opened up from JFK to LHR between around the 10th of Aug and 18th Aug. So we are now booked back on club. But I've been looking for WT+ or Club for months for the way out and not seen anything to JFK during the period we need.

While upgrading the return today over the phone (changing an avios booking) with BA today I asked about aircraft loads and they told me that from now until about the middle of next week the routes to the USA are very busy. The flight we are currently booked on is showing 9+ available seats in each of WT+ and Club, and 8 in F and only 4 in WT on Flight stats but the BA operator told me there were only 4 seats in WT+ and 6 in club available. Not sure why this should be so different to what Flight Stats shows, maybe the flight is well oversold in WT and they are keeping options open.