Leaf Problems

You're nutirents locked, The pH and EC should be measured consistently throughout the life cycle of the plant and adjusted based on the stage of growth. If you are taking measurements regularly then the problem can be caught early before the plants are affected. In soil the salts and pH imbalance can build up. In this case the soil should be flushed with slightly pH adjusted water until the point that the runoff is similar in pH. The medium must then allow for drying to prevent root rot, and then start watering with a fresh mixture. Hydroponic systems can either be adjusted with a pH supplement to adjust up or down. If the situation is more difficult than just adding pH supplement the system can be drained, run with distilled water to run through excess nutrients, and re-balance with a fresh nutrient mixture.

https://dutch-passion.com/en/blog/a-visual-guide-to-cannabis-deficiencies-n987

It's a fairly desperate potassium deficiency ... a pH of 7 is NOT helping her out and not feeding her except for that 20-20-20 (which has about 18 too much nitrogen) is further impacting her... Get her pH down no higher than 6.5 (I prefer 6.2-3), start feeding her something that has a lot more P and K in it than nitrogen ... something akin to 2-45-28 ... this should have been started when she was at about week 2 or 3 of flower but she'll welcome it even now... Those damaged leaves are not going to get better but the buds will respond nicely to the better treatment and any new leaves appearing should not show the deficiency... Good luck...