I'm not a hydro guy, but isn't 6.4 too high for hydro? You say your 6 weeks into flower? According to chart at 6.4 Phosphorus isn't available, and in flower the P is important. Don't worry Lex, for sure someone on here know's. Good luck man.
My guess is it’s an iron or sulfur def caused by ur ph being to high, if it was nitrogen the bottom leafs wouldn’t have any green to them at all cause that’s where N would be pulled from first & seeing how the top is the most effected it tells me it’s a trace nutrient def which is most common with a high ph like you said u had.