Looks like a standard over watering issue, my friend. Medium is saturated by the looks of the picture and the seedling symptoms are screaming "No oxygen". With any of the girls showing these symptoms, I would back off watering heavily and localize anything you give 1-2 inches away from the stem. With your water saturated soil, you can gently till the area around the seedling to get aeration to the soil and help disperse all that extra moisture (Top 2-3 inches). If you mix in perlite or the like along with the tilling, this will expedite the process and the moisture will dissipate faster.
If push comes to shove and water retention issues continue, consider moving the girls in the plastic pots into fabric, as well. I'm personally done with plastic as I had constant issues with soil-water retention regardless of how much I conditioned my soil. Fabric is the way to go, and if you make the move now it'll save you some stress in future. Trust me.... (Or air-pots. Also higher aeration but fabric is cheaper).
Anywho, good luck. Everything will be okay and I hope you know that👍. This happens and isn't the end for them. Keeping it in this saturated state will eventually kill her off though... Couple tweaks and some conditioning and all will be well. Cheers and happy trails.
