Seedling stopped unfolding open

it's probably just a slow down... try a substrate with better aeration for faster root growth nex time... looks dense and not much perlite or similar for a more optimal level of o2 at roots... closer to 50/50 is better for soil and root health, though it does mean you have to irrigate more often. it's also in a smaller pot which will impact size compared to others... though it really shouldn't stall even in soil at that size.