It looks like a boron or phosphorus nutrient dificiency cause by nutrient lockout. Your TDS/EC readings are to high for that stage of growth. Flush out your soil with a couple cups of plain water(RO water preferably). You shouldn’t need to use much nutrients if any at this stage. When you do use nutrients start off with 1/4 the recommended dosage and move up from there if you start to notice other dificiencies. The only other thing I can think of if it’s insect related is check for thrips. They sit on top of the leaves and scrape away at the leaf. Use a jewelers loupe or magnifying glass to get an up close look. Hope this is helpful. Good luck on your grow!