It could be heat stress. what are your temps, does it spike. Dont hve the lights too close

BTW
the little round leaf is called embryonic leafs.
and the normal leafs are called the True leafs

the plant has three stages as we all know. The seedling stage ends when you have 4 nodes of true leafs and the 5th set is very small but visible in the center.

I am not familiar with the soil you use. maybe read about it. Make sure it dosn't have manure or ferts in it. you wana use something like a mix of peat, perlite and sand. Lots of perlite like 50%. you can go up to 100% perlite if you wish. i haven't

I say check the heat.