leaf turn pale green and fall off

It’s called the Die-Back , the plant is moving its Nitrogen from the bottom to save the top. It usually happens around week 3-4 of flowering if the plant didn’t had enough Nitrogen during the stretch or when new grower is switching too early between Veg to Bloom nutrients. (Switch from Veg to Bloom nutrients in week 4 of Flowering) Adding Nitrogen is too late now, roots are only processing P and K now for the buds. But don’t panic, just keep feeding it correctly with its Bloom nutrients and wait for the leaves to turn completely yellow before pulling it off. The plant will do the rest of the job.