Wild guess: maybe you've got a seed-transmitted virus in those plants.
On the off-chance it's a seed coat virus, coat the seed overnight
with a thin film of skim milk (yes, it sounds crazy, but the milk proteins
can inactivate quite a few seed-borne viruses) and/or dip your seedlings
in skim milk (room temp) before planting. And change the soil in the
basket.

Kay