What's the issue?🤔

Agree with below comments. Over feed, over watering, pH is out of range..

The first problem is your diary, you’re writing it like if was a story or a fairytale’s…. There’s no information on watering dates , EC levels , light intensity, to make any accurate diagnosis… Then the second problem is using NPK fertilizer since the first week is not recommended, particularly in fresh pre-fertilized soil… Your stems are already purpling, this could be from overwatering or too intense light. Leaves are telling like if it was overwatered or too intense light or an excess of Nitrogen …. If you want an accurate diagnosis, stop writing about your plants life and start to put some information like growers do… In any case, your seedlings needs to start recovering.. back up your lights to reduce the photosynthesis for sure. Give them some roots enhancers like humic and fulvic acids , and some mycorrhiza and trichoderma fungus then let the soil dry to make a wet and dry cycle (lift your pots after watering , then lift again after 5-6 days)

nute imbalance or poor watering habits.. the latter would probably take longer to show up than this early. usually when it progresses from the serated tips, it's K-deficiency