have you ever seen a plant do this

Hello mate, i think your plant it's in trouble, it seems to be affected by root rot. Probably you gave it too much water. Try to wait that she drain the soil but i don't know if this process will be reversible. Good luck and happy journey!!

hey 👋 Before working with coco substrate, I often had the same thing ; with autos, once they settle into their final pot, during the first 2 weeks you can feel that they’re trying to extend their roots while still being fragile and sensitive to nutrients; as a result, the first and sometimes the second pair of true leaves show small burns. But your seedlings in week 3 and the following week are, and will be, healthy ; the plant is now adapted enough to handle the substrate 👌 Think about CalMag, and if you have a root booster like Voodoo, don’t hesitate to give it a few drops 👍

Yes. normally from roots not getting enough oxygen and rotting. But see what others say.

Yeah it's scorched from nutes but the new growth looks fine so no danger now.

Yes.