Top leaves yellowing, lower leaves are healthy green

Light yellow tops and fan tips could be a sign of heat/light stress. If that's not the case check for overwatering and wet roots, blocking nutrient uptake. Since they are in early flower, you might be missing some nitrogen if none of the above applies

https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg Hows your PH and sulphur intake on the plant. Could also be a buildup locking out things. ever gotten a runoff reading? or ever watered with a bit of runoff. if not that is likely the cause. Good Luck!