Brown Spots and Vein

You don't give nutrients to a seedling, as they have pretty weak roots and even might be have just a single root hair and no more than that at that stage. Even if you think it's diluted, that's only relative to manufacturer instructions, so it will be hot for your very young plants. Moreso, I doubt any manufacturer recommends feeding a seedling. There is no other cause, but I wouldn't call that a bad sign, not at all, your girl looks amazing! Please keep on the great care, and allow her to have a bigger green mass and more leaves before feeding, she will thank that later, now you're only wasting fertilizer, even if that's just a drop. As a recommendation, for this stage the best feeds ever are plain rain water, and californian worm compost tea (or any worm casing tea). Those are usually 0-0-0 or 1-1-1 and won't never burn your plants, while completely filling your soil with the right bacteria and fungus your roots are craving off. Happy smokes!