It’s natural, it’s part of the life cycle of the plant, during flowering the plant moves her nutrients to the flowers, it’s been like this since millions of years. Reaching the harvest window with green lush plants implies 2-3 weeks flush, meaning too many nutrients were used, which impacts the final quality of your weed…

First thing I would do in your place is check the EC of the runoff and how much EC you’re feeding the plant. The plant doesn’t look like it’s suffering from a Cal-Mag overdose that would suddenly cause this kind of “browning.” You most likely have a nutrient lockout, and the plant is generally overfed. If you’re using mineral fertilizers, I would water it with plain water at the correct pH from time to time, without nutrients, to let it “rest.”