I think they're just ready for transplant and fresh soil. Your ph looks good, light distance good, overall leaf color healthy green. The only question is your heat. ..it says 78 for day and night, is one off? If the heat is high or the lights too close it will cause this yellowing, as will light starved leaves...but none of those seem to be factors, just possible low nutrient levels. Watch for changes after they get that big batch of soil.👍