To me it looks like a high ph issue! It also looks like light bleaching but considering ur light is over 20 inches away... the 7.0ph would highly likely be the culprit! 7 is rather high for cannabis plants in soil. Try n reduce ur ph and it should stop the spread and minimize the damage. Considering u still have a couple weeks left of flower u definitely should do a good flush and than follow up with a reduced feed. Happy growing and stay lit fam