Hey there BRL, you’re right that at this time in the flowering the plant needs K. But the yellowing is still magnesium deficiency . Same reason as before , and the reason for that is that there is a fair amount of “inertia” meaning it takes some time for the problem to stop spreading , the plant to recover , etc. You might be better off reducing yet further the light schedule , or try to muscle through by increasing a bit more the cal mag ... It's a tough thing to bounce back from a mistake on growing , that's the problem. Hope this helps.! 🚀