Fox farms mediums are pretty potent straight out of the bag and don't usually require any added nutes for a while, maybe well into flowering... I've never tried it so I can't speak from experience. I have noticed that, during the transition from veg to flowering mode, the metabolism of the plant or something changes when using a lot of nutes... almost like nute burn suddenly appearing for no good reason. Perhaps this is what you are seeing. Nitrogen is needed for both growing modes, and large fan leaves often yellow and die during the later stages of growing, starting at the bottom of the plant... again, I don't think this is an issue because FF is pretty well stocked.
Leaf damage doesn't usually show up right away and can take a few weeks for some damage to become apparent. Maybe your earlier overwatering issues caused some damage, since new growth looks pretty good to my old eyes. A PPM test of any overflow will give you a better idea of what your nutrient levels look like, but addig additional nutes, depending on what and how much is added, can throw off the balance of nutrients that are needed... with the emphasis on balanced. A slurry test will help to determine your soil Ph and a PPM meter should give you an idea of the state of the soil. Between these two, it provides a lot of information that might be needed to fix this possible issue.