Curious brown spotting

These plants are in the third week since first leaf. The brown spots have appeared on one of three plants, including the first two leaves. Same medium, same water, same light. The new leaves look great, but with time, the one plant‘s older leaves develop brown veining.

I just gave them a pH 6.6 limited watering with Recharge. Next water with some CalMag. Perhaps it’s just a genetic screwup. The other two plants look great. Going to hope the weakest plant can grow out of it.