Do you have enough phosphorous in the mix? Sometimes it also looks like a calcium deficiency due to how the two interact. Could be a root pH issue.
This is from an internet page but it saves a lot of typing:
<i>A phosphorus deficiency tends to be more common after plants start making buds in the flowering stage. Cannabis plants tend to love phosphorus in the flowering/budding stage and it is unlikely for a cannabis plant to get too much phosphorus using standard nutrients formulated for a flowering plant like cannabis. Nearly all flowering nutrients will come with an abundance of phosphorus for your plants. So if you’re seeing a cannabis phosphorus deficiency while using standard cannabis nutrients, chances are you actually have a root pH problem (explained below in the solution section)!</i>
Phosphorus deficiencies cause dark splotches on leaves and can appear at the bottom of the plant on some of the oldest leaves. Learn to spot & fix the issue.
www.growweedeasy.com
I am most curious as to what your root zone pH is running. It would explain everything if you had a pH problem there. Runoff isn't reliable. I use the Blue Lab Soil pH pen
https://www.amazon.com/Bluelab-Pensoilph-Soil-Tester-Pen/dp/B00CJIDFAS
That is what I consider my MVT (Most Valuable Tool).