I recently noticed my Uchiki Kuri squash seedlings were not progressing equally; while one has dutifully produced a healthy first leaf and is making a start on its second and third, the other has no growing tip.
This is called a 'blind' seedling, and occurs from time to time (especially in curcubits, in my experience, but also in tomatoes, peppers, brassicas and virtually any other kind of plant), seemingly at random and for reasons unknown. It's a disorder or defect, rather than a disease. This blind seedling has no true leaves at all, but sometimes a seedling may develop just one true leaf and be left with no growing point, like the tomato seedling below, or even grow a full pair and then stop.
Sometimes, if you keep it going long enough, the blind seedling may sprout a growing tip after all, but it's much quicker and easier just to compost the plant and resow. And that is what I have done.