I noticed today (day 6) that one of the cotiledon ...

Your plant shows stress pre-signs. We're missing some informations to clearly identify your issue, but here are some clues : this could be related to overheating, I see your daytime temperature is 32°c, which is very high for a such young plant (and even for adults). A nutrient burn could also be responsible for this, but we don't know what you're giving to her and at what frequency? Finally, this may happened because of the proximity between your lamp and your plant : 15cm is very close, once again you're working with babies here and you don't want them to be hurt. I would try this : increase the lamp distance, decrease nutrients (only plain PHd water until the next healthy leaves appear), and find a way to lower down your day+night temperature. I'll be around, good luck with this baby!