What's going on?

plant looks healthy. node spacing is fine... Probably a misperception. Roots might be stealing more resources at the moment than what you can see growing above ground, just one possibility. Our eyes are imprecise and inaccurate tools. Maybe, VPD is too low? VPD is based on leaf temp which is a few degrees cooler than atmopshere, and an even lower VPD. Reduce humidity to maybe 60%? maybe 55%? You are techinically still in seedling stage, but it's not as delicate as an initial sprout. Possbily ready for a faster rate of transpiration. Leaf temps are -2C or -4F, give or take, due to evaporation taking place (cooling effect). If VPD is too low, then not enough nutrients can be taken in to keep up with photosynthesis, which could potentially slow growth.

please dont use cardboard , could led fast to mold and is bad for humidity and you dont know what is allready in it

Hasn't grown since yesterday haha, funny made me laugh that 1.. She was probably on a good wet dry cycle and suddenly you've give her too much water, killing off the roots, suffocation, dry is as important at wet.. Good luck

that really isn't a compatible nutrition program or combination IMO... unless (i) your chosen products are solid salts or (ii) your chosen products have solubility/precipitation issues with tap water, you should likely steer clear of RO unless fully able to touch on what RO water is, what the difference are to tap, the pros, the cons, and the true nuances... you gave them unstable molecular water and a microbial inoculation, and didn't mention anything else that would provide nutrients or life for the plant...the microbes need food at a bare minimum plant looks more moist than it would like to be, which brings light intensity but also just plant health into question. why is the plant not reaching your expected water uptake? were you wrong or is she stressing?

is it soil or coco ? if 2nd then u shouldn't water with RO water😗

hey 👋 Hard to say... The soil may be too wet, and the light intensity a little low