is it on the buds? or on the leaves close to the top? From what i noticed/understand, rot isnt just browning, its a gooey sticky mess, a moldy gunk, thats not exactly what im seeing here.
How far into flowering are you? have you checked the trichrome colors? keep in mind, if its just the leaves, the plants are sucking away all the goodness from them to stick in the nugs, and more (less likely) deficiencies appear