Was exploring London to Tokyo with Avios, and came across a comment on Reddit saying "book from Madrid". So I searched (on the BA "book with Avios" page) for Madrid to Tokyo. This gave no direct route, but did give an option via Paris. OK... please search Paris to Tokyo. Again, no direct route, and this time the option was via Madrid! I think the Paris route was JAL, and the Madrid route was Iberia.

What is going on there? How do I have a chance of finding availability for a popular route? SeatSpy only shows BA metal, and is very limited for anything not originating in the UK.