Could you post a picture of the plant in the setup? I agree it looks a lot like
Calcium, Nutrient Lockout or
PH from the pictures.
Calcium Deficiency



You are also said you're using GreenTree Growers' Blend, with is a
Coco-based medium. Cannabis plants grown in coco coir tends to need more calcium compared to other growing mediums. Calcium deficiencies are also very common when using RO water since it strips out all the minerals that are typically found in tap water. With both coco and RO water, I highly suggest using a Cal-Mag supplement. You mentioned you use Advanced Nutrients, here is their
Cal-Mag supplement. Advanced Nutrients will definitely grow great cannabis all by itself. Once you get everything dialed in you shouldn't get deficiencies with it.
With Calcium, a slightly higher pH seems to help, so 6.5 in hydro, or up to 7 pH in soil. I would mention that although you listed soil as your growing medium, you are basically in a hydro setup since your medium is mostly coco, peat, perlite and Happy Frog (a gentle mix for seedlings), plus an initial charge of nutrients. At this point, your plant has likely used up all the nutrients that were initially there (you don't have slow release nutrients like amended soil), so the roots are only getting what you're giving in the water. That means pH and nutrients are especially important. Are you also checking the pH of the runoff water, to make sure it's not way higher or lower than what you're putting in through the top?
How close are your grow lights to the tops of the plant? What's the temperature in the top canopy and is there a good breeze? Too-close grow lights can make these symptoms worse. If the leaves seem worst near the top of the plant, I'd consider putting the light up a few inches until the plants start getting better. I know you want to maximize light in the flowering stage, but slightly lower light levels help plants recover from problems.
I hope that helps, a few spots on your leaves never hurt anyone and your buds are coming along great!