ive have no personal experience with this problem but i would flush soil with 6.2pH water. and after the soil dries start feeding at half the strength you were. working your way up again slowly.
oh and i grow in soil and never used a ppm meter. i think ppm meter is more important for hydro.
the leafs damaged will never recover, as in wont go back to a healthy green, if new development isnt getting these problems you solved your problem. if it continues to spread after flushing with 6.2pH water, then i have no ideas.