hi man, could be 1 or several things.
looks like a nute deficiency, are you using any food yet? As if not its maybe time to start a half dose at this age, if you are feeding them, could be slight nute burn, but usually tends to affect the tips first. Is it nute-rich soil?
give them a flush to eliminate toxic salt build up. then feed once 2" of top soil is dry, as remember they need air as much as they need water, or u could see rot root, hence stunted growth.
Let your water sit out before feeding, and make sure you pH test, this will stop chlorine burn to your plant.
also if you are spraying leaves under lights, any water droplets can cause leaf burn due to heat magnification. What wattage/type bulb are you sticking them under, could be too strong for seedlings?
have you got a fan giving them fresh airflow? if not id say get 1 in there as u dont want mould due to high humidity.
hope you finger the problem soon, just go through all possible scenario's and you should get it right mate..
Good luck.