They are locked out of nutrients, possibly due to hot soil, lack of oxygen in the soil and overwatering would be my guess.
I recently had the same problem growing seedlings in Thai soil that was ready-mixed to grow cannabis. I foolishly thought I could just let them grow in soil and water with low maintenance, top feeding now and again. Within days of emerging they started to turn yellow and were locked out of nutrients.
I repotted them in my coco and perlite mix and fed them synthetic nutrients, within 48 hours they were back to life again and a lush green colour. Now they have doubled in size in 24 hours and going great guns.
I have had far better results with coco than soil here, even when have premixed the soil myself.