Grow northern light

After reading your diary and checking the photos I would say this is a phosphorus deficiency. your ph is on point so that's not the cause could be the temperature changes. Some strains of cannabis use much more phosphorus than others, or can be more susceptible to a phosphorus deficiency, and you may have many plants in the exact same setup with only some of the plants showing signs of a phosphorus deficiency. So this one just may need its own feeding regiment to make sure it's getting all it needs.