PH is in good range, Is this some sort of lockout or micro deficiency since the lower leaves aren't showing any signs? From the looks of it, could be

Week 2-3 of flower, near the end of stretch and the upper leaves are this bright green color while the lower leaves are still dark green. Used some left-over biobizz grow the last 2 waterings, did not change anything for the past week so it can't be N. Plant is otherwise vigorous