The roots look fine so def not that. Obvs dont transplant for couple weeks now as you will cause alot of stress by uprooting. Ive grown 10's thousands plants in coco and theres not much i havent seen. I have never used that light but im wondering if its just a bit intense for the seedling.
Ive also never used coco with cork in and i honestly dont know how that will affect the medium. I will never pretend to know if i dont, thats new one to me. Id just get diff coco and coco with the atami bcuzz if available or plagron. Avoid canna coco if poss or unbranded cheap coco. Justfeed enough so pot is light to lift after 24hours. That will stop any over watering.
Just follow the regime and that seedling will pull around im certain of it. Run ph at 5.5 for now as its a better range for veg.
By the way my podcast will def start sun and will be called 'igrowcoco'. Someone said i was no1 in world yest, im not sure about that lol but there aint too many people have the understanding of coco i have. Even though i hate canna nutes it should be ok with calmag added and its normally in flower that it causes issues. Keep ec at the values i said and i reckon 2 weeks from now you have a nice bushy plant. Just dont have led too close.