How much runoff are you getting? Maybe too much? Also leafs sometimes seem to do this on the beginning of flower so maybe some pruning will help you out here. You should be shooting for 120ppm of nitrogen for both veg and flower so if you have that then I don’t think it’s too low n but something could be locking it out. Also I don’t think they are root bound especially if you’re feeding multiple times a day, you can grow giant trees in a 6” cube with no root problems. Maybe you’re overwatering a bit, which may be killing the feeder roots which maybe is causing some root bound issues? Also did you say you’re feeding at night? That might be a source of the problem unless I’m reading it wrong(which is definitely possible)