Hey Growmie ...
With you making those adjustments that you stated I would definitely say it's your soil causing the issue. It seems to have too much nutrients in it for a young seedling. The plant will be fine just let it grow a bit so it can intake some of the nutrients that are in the soil.
Hopefully, this was helpful and remember ..."Happy Growing" and "Free your mind one puff at a time".
Mr_Weeds_Autos.