I keep hoping someone with better eyes than mine will answer this question for you.

Honestly I can't see what if anything is wrong in your picture. Sorry.

Bu now they should have grown quite a bit so perhaps a new picture with a close up of the problem could be posted?

Dave

Sounds like damping off. Seedlings are exceptionally susceptable. Several common fungia are capable of causing damping off, including Rhizoctonia, Alternaria, Sclevotinia, and water molds Pythium and Phytopthora. Symptoms can include rotting or wilting of seedlongs, leaf mottling, stem leisons that cause the stem to turn black, and appear waterlogged. Plants often die off in a circular pattern as the fungi radially from the point of contamination.
my advice is to purge the already infected seedling, and the adjcent plants to stop the spread.