your pH is too low for soil, and could be the cause.
have you been feeding? if so, stop. Seed is packed for 2 weeks and soil has nutes to start too -- varying amounts.
just becaue 2 other plants handled the same environment doesn't mean anything to this one. it doesn't like something for sure.
so, if you have been feeding, ease off for sure.. water-only and pH'd to ~6.8. A slightly higher pH wil help with N and still in a normal range for soil, otherwise.
if you havent fed at all, i'd still be hestitant to react to this by tossing stuff into soil without knowing for certain. soil should still have plenty of nutrients still.
i don't htink it is, but light burn? it would turn white after yellow. a plant that young would fall over, though, so thisis low on list of possibilities. nodes would have no internodal spacing etc.. you'd be able to recognize it, if it is.