I see this condition in the majority of my grows, however I don't recall ever seeing it when growing in the sun, which is obviously more intense then these led bulbs.... Something in the spectrum of these bulbs...… I have never found this to have ever effected my end product. You are good to go. I will say, I prefer a 18/6 as opposed to 20/4 and even 24/0, at the stage you are entering...… and especially when the young plant is on its 4th to 5th set of leaves. They need rest, and you will get crazy growth in the darkness. They go through a lot and use a lot of energy during the light cycle activities, the use the dark cycle to heal.....and grow......