I don't think it really is a case of fullbacks "figuring him out", because he's like, the fastest man on earth, so unless someone has figured out how to run faster, then Valencia has what it takes to play to his strengths.
Valencia had been playing in the Premier League for 3 years before we signed him, doing the same thing, knocking the ball past the left back, bursting past and getting in a cross. He continued to do it for another 3 years for us, before having an iffy season. It didn't look like he'd been figured out, rather than he barely even tried to do what he normally does. He was far too indecisive, and didn't make many efforts to burst past anyone. He looked void of confidence. If he'd been figured out, we'd have seen mroe of him at least trying to take defenders on and getting stopped. He just didn't even try.
If he has been figured out, did it really take everyone 6 years to realise "Oh, I see. He knocks the ball on, and chases it. I got it now" and then that was him completely sussed? I think there's more to it than that. Trying to keep him from going onto his right foot doesn't always work. His acceleration is the best around IMO. The defender would need a slight head start, and that gives him space to play a pass inside, or even run into the box, which he has often done, before he became void of any confidence to play his natural game.