The true leaves of my roselle seedlings are brownish at edges and discolored.
Something happened during the time of the 1st and 2nd set of leaves. Too hot? Too cold? I can see some sort of insect infestation, can't see very well but maybe white fly or mealy bug? That would account also for the 'sucked dry' of the first leaves. I don't see them on the new leaves. Look beneath the leaves just in case.
Cut the dead/dying leaves off at the stem, do not cut the new growth at the top of the trunk or main stem of this plant. Think of this plant a tree. You do not want to top this tree. One takes the branches off right at the trunk.
Looks to me that you are watering too much. I am glad you used potting soil I hope there are drainage holes, bottom of planter/pot raised slightly off the surface where it sits...an air layer between pot/planter and the surface that supports it... and absolutely NO rock or gravel below the soil and above the drain hole.
Your plants below the first leaves look extremely healthy. What are you doing for fertilizer? Do not add any more fertilizer for quite a while if at all for your season...where is it that you live? Cut back the water. Don't water until dry. These plants are succulents they store their own water and too much water will start root rot. Try to focus watering the root zone, wet but not sopping and slightly watering the rest of the soil where there are no roots as yet to suck up the water. Roots will drown and rot if the pore spaces hold water too long. Roots keep the water moving and draining. No roots means a water logged bog and drowning roots of plants.