Overfertilization? What could it be?

Hi there! Taking a look, it seems that only some lower leaves are yellowing, and that the plant is in early flowering. If it was overfertilization I would expect more growth to be showing issues. Especially on the leaf tips. It is normal for lower leaves to be used for energy by the plant at this stage. The plant is redirecting it to its new priority, bud formation. The plagron substrate may not have quite as much nutrition which is why you are only seeing it on those plants. Or it could be a generic variation with plants grown from seeds and those two happen to be a bit hungrier. As long as the upper leaves remain healthy and green you should be good to go. If I missed something, or you have any questions in the future, you are welcome to shoot me a message. Happy growing! 👍🌱