Stunted Seedlings?

Looks like seedling with wrong ph in the pot to me personally. No info about gtow, so just wild guess...

The plant hormone auxin is responsible for cell elongation in plants. Doubt it, but it could be genetically pre-disposed to repress auxin or change levels required to function as expected. Phosphorus is essential for seedling growth and development because it's a key component in many processes that plants use to create energy and build biomolecules, nutrient responsible for cell elongation and division. P content in seeds has direct effect on seedling vigour and crop establishment as it affects root development and associated physiological processes. 10–20% reduction of seedling biomass and P uptake was noticed in low P seed germinated in P-deficient soil. Who knows could just be as simple as soil compaction from flood watering from the top but since you have grown a bit and have not experienced it before. Vpd could certainly help too, how accurate is your RH% is it canopy level? What's night Rh?

As long as some sort of stem develops as those new leaves come in, you are fine -- too much stretch is bad, but so is too tight and that leads to stunting. that 2nd node (or maybe third depending on how you count it) usually starts to spread out a bit at this point. An internode should form soon (2, technically) could just be slow growth. a proper VPD can help quite a bit to speed up seedling stage.

Are you re-using the substrate or is it a brand new mix?

hard to say, no diary. just give em some more time. Normally this can be caused by the medium or the feed or the lack of light. So I'd start there for trouble shooting. also ask. what is diffrent in this run vs others. seeds? pots? schedule?