I just made a booking on a 777 many months ahead. The J cabin was empty except for two seats. I booked window 12A with no-one next to me.
When I checked on MMB two hours later, I see that 12B is now occupied. Of course, it could be that someone booked the seat in a two-hour span. But all other window seats are empty and I wonder if BA is blocking it because I am Gold.