P is used for transportation and Energy... So you have High Transportation rates at the top/upper canopy.. The lower parts look less effected also smaller leaves, Larger leaves=higher rates of transportation.. A General rule when problem solving feeding/balance issues.. Is if it's lower on the canopy it's a deficiency and if it's upper it's from too much... This is only a kind of sort rule.. It doesn't always apply to every situation.
Looks like a pH issue. If it started at the top of the plant, perhaps add some CalMag.
Just remember that once its stops spreading, you're good. These spots will never be green again and that's not a big deal.