From my research it could be phosphorus toxicity, iron deficiency or sulfur deficiency.
Or it could be over watering.
Or it could be poor drainage.
Or it could be light deficiency (I have a 42watt Tled with some led strips until Mars 600 arrives)
My best guess would be overwatering. The 42w should be fine for the seedling, but I'd bring it closer as well. She should bounce back soon, just be sure not to overwater and keep a good soil pH and you'll be fine 💪 happy growing!
So I found out that the soil was pretty toxic for them due to some silly fertilizer so my hands were tied to perform a bare root transplant. Hard for them but it is the best way to save them. Wish me and my babies some luck