Leaf deficiency

Sometimes a potassium deficiency (like all deficiencies) can be triggered by stressful conditions (overwatering, heat, transplant, etc) and may clear up on its own after the period of stress is over. If you only see one or two affected leaves near the bottom of the plant, and the problem isn’t spreading, I wouldn’t worry too much about it! If you've determined this ISN'T caused by overwatering or heat or any other kind of stress, what you're going to want to do is to make sure your PH is in a good range for your plant to absorb the potassium you're already giving it in the nutes you're providing... so check your runoff... it should be no lower than 6.0 and no higher than 7.0 (but weed generally needs to be right around 6.3 for optimum conditions. If you're out of that range, you'd need to do a flush and get that under control. My only other thought here is that this may just be the natural progression of a grow that is nearing it's end since I'm not seeing that you have a BIG problem here... i.e., not all leaves are exhibiting the same thing... If this were my grow, I'd sit back for a week and see how things progress. Nice looking grow, good luck!