the substrate isn't saturated constantly, right? unlikely damping off based on pictures. that is microorganism infecting, weakening stem and it should be mushy or slimy if it is damping off.
so, this is more likely light intensity.. more shade.. it's just a seedling. too much light can absolutely make a seedling flop over like that while looking fairly healthy otherwise, which yours does.
try keeping it indoor for a bit longer, then baby it a bit more as it is exposed to sun and nature. slowly expose to more.