I grow some little dahlias from seed too, but I haven't tried the Figaro. This year I noticed a few of them with yellowing leaves. I don't know if it is a nutrient issue, or watering issue. I know some heat loving plants can't take up nutrients easily if it is colder than optimum, especially if it is cold and wet. I moved mine to my top light shelf which is my warmest spot, and am not watering until they are quite dry. They seem much happier. I fertilize with 1/4 strength fertilizer every time I water.

Looks like you have them in the same tray as broccoli ? Broccoli and dahlias have very different needs.