Sounds like the soil is too "hot." Anything over 72F can impact apical meristem growth at such a tender age. It's vital to make sure humidity stays high for seedlings; when their frame is tiny, they have extremely limited water reserves, and even a slight loss of oxygen or heat spikes, even for a short while, is enough to fry or severely stress delicate teeny tiny root fibres/hairs.
The single important thing for a seedling is maintaining a consistent environment: no spikes, no droughts, just warm, wet spring days; a well-draining medium.
Soil temp is often overlooked. Don't allow it to go over 72F
If you apply 0.1-0.3ms/cm airflow, you can then sit the small pot in a catch basin of water under the soft light. This will allow the water to evaporate, prevent stagnation, prevent soil compression (bottom wicking), and prevent drying out of soil while the seedling establishes. Helps me a lot.