I am having the same issue with some of my 10 day old babies in DWC. The ones in the middle of the tub, and also the most developed, have their first true leaves curling like yours. However, the ones on the perimeter, in the same nute solution (~300ppm / 5.5 pH drifting to 6.5), look fine. I originally was thinking my nutes were too strong or the plants were too wet, but it's going on the third day with the inner curled/outer normal pattern, so I'm thinking maybe it's light intensity. It doesn't seem to be holding back growth for me, so I am leaving the light where it is.