I’m using BioBizz light. I don’t know about coco perlite but for soil and this nutrient line 6.3-6.5 is where you want to be. Also wouldn’t make sense why only 2 would have weird things going on if the other 2 are also watered at same ph. All these are all same strain.
Put a lid on them for a few days starting right now...anything below 50% rdh is too low for seedlings. Even 60% is kinda low. Usually, you'd keep a dome on them until the roots have set properly, usually noticeable by noticeable growth in the seedling.
Mospeada Nailed it! RH is way to low. a dome and heating mat will do wonders! I thinks thats what's causing them to suffer is mainly your RH. I've had some in the 70-80's, lol and they did well.