I have 2 new, 2 week old seedlings in my grow room and the bottom first set of leaves is yellowing at the tips to like half way up. Their not as green as the new growth.
Looking at your diaries, the biggest problem I see your relative humidity. Those little seedlings need around 65-70% RH at this stage of their life. Along with 250-300 PAR/PPFD and a proper watering schedule. Provide those three things and they should bounce back. Also maybe consider up potting those plants in the next week or so.