More info will help.
I searched up your light, and from what I can find, they claim it will replace a 250watt HPS and flower a 3.5'x3.5' area. This is exaggerated.
If you had that over a 3.5x3.5 you'd only be getting around 20watts per sqft of blurple light.
While watts per sqft aren't entirely accurate, without anything more to really go on as far as light efficiency, this isn't enough.
Maybe if you had it over a 1.5' x 1.5' or 2x2 to ge tthe wattage up.
The others saying it looks burnt contradict what I know of FFOF, which if you aren't feeding much, should be low on nutrients by week 12.
It usually starts off too hot for seedlings, but is fine for veg and flower. You state you are well into flower, and have not fed it much at all, so I'd think your plant is starting to feed off itself (dying fan leaves).
I think your solution lies in more lighting power and a better feeding regimen.