tl;dr if you were unaware, Lee Sin can cast his R in mid-air if he uses the second part of Q while in the middle of a W. The end result is that he gets a guaranteed ult off with no counter play. If he just takes Q without cancelling a ward hop, his R just queues and doesn't go off until he lands.
You can't flash or use mobility to escape this, because you're rooted during the casting time. Because the cast time is now occurring during his Q, it's impossible to Flash or use mobility between Q landing and the ult. If you Flash before the R, he continues to follow you with his Q.
9/10 times I have trouble with Lee Sin as an immobile mid laner, it's because they're exploiting this bug. I thought they were just BM'ing until I realized how this works. This should really be a high priority fix.