I also think its potassium related. Looks like a combo of potassium and Iron def. Could be caused by an overconcentration of calcium from overfeeding. Could also just be light stress from your light being too close.
I would raise the light to 18", flush out the soil with water until ppm is below 1ec, then recharge with a balanced nutrient at about 1.2ec.
Also, I hate to say this but since you're using
promix... Check for root aphids.
When you see issues on a couple plants but not others, often there is a soil born pest to blame.