I believe this is relatively normal. It is sometimes caused by a Nitrogen or Sulfer deficiency. If the leaves are starting to yellow it is most likely Nitrogen deficiency. Sulfer deficiency is seldom deficient because many fertilizers have some in them and cannabis plants don't need much. It seems to me like you are using a fertilizer with a good amount of N so I think it is normal. I am a noob too so take this advice with a grain of salt.