In my zone…or maybe just in my garden…putting out plants in August is the kiss of death…even with lots of spraying and other protections from sun, weather and bugs. So all gardening is local.
My soil/air temp is too hot July/Aug to get good germination of brassicas. I sow into small trays mid July (inside) to get good germination and then prick out into cells 1st of Aug and grow on outside in small protected area. At the 6 week mark (~Sept 1st) when there is room in the garden and they can stand the assault, they go out.
@barry to me this looks like a combo of root damage and too high lighting where the leaves cannot get enough moisture up to meet the heat/light load, so the edges wilt and eventually the whole leaf. I find small plants do not need ferts, some of which can burn the roots. Some folks are also concerned that their brassicas don’t get leggy so they supply lots of light…and maybe over do a little bit too much. Well just my thoughts.
Edit: a few other thoughts. To avoid legginess you do need lots of light but it needs to be cool also to avoid too much transpiration from leaves to cool off. Also if your soil is recycled and not solarized, there may nematodes in it which a root autopsy may show.