So OP, what did you find out? I'm having the exact same issue or atleast it looks exact. At first it appeared as only Mg def (lower leaves), so I treated with 1/2 tsp epsom/gal (only used half gallon and monitored to see if rest was needed; wasnt). Then came the P in middle of leaves (lower leaves). Kept eye on it for 2 days, then it was time to feed anyway, so gave 2-7-7 mix (same K cause slight def started low. Still progressing. 2 weeks into flower, therefor signs of N def (normal), P, appears Mg def is reoccuring; P def is affecting not only the bottom, but mid, and now starting on top 2 days after feeding. pH has remained at 6.5-6.6 throughout entire grow. temps have been 75-80 (i let it go to 80 sometimes a day after watering to help dry somewhat; helps keep humidity between 40-50%). Water and feed has been 6.7-6.8 going in, and runoff/soil have both remained 6.5-6.6. Other forums keep saying pH, but I've had no problem with any other plants nor has pH ever been out of wack for any of them. Same medium, same environment, same strain. Everything identical. Except that this has turned out to look like my best plant ever until this.
I'm about to flush in a few hours if no solution is found (been at it constantly since it started) and then replant in new soil mix. I'd like to think I'm an intelligent human being, but if it sounds as though I've been wrong in my diagnostics, I love to learn. Educate me.
Not trying to jock your thread, just find it relevant for info gathering all around for anyone with same issue.