Wellige Blätter

Also, wavy leaves and those yellow "stretch marks" come from uneven nutrient uptake. This can be caused by overwatering or incorrect pH levels. The pH should be right if your values are correct; you can easily raise it. With such young plants, it's easy to overwater; it probably only has a taproot right now. This is most likely the reason, since you planted directly into the final substrate and are watering very generously. The plant isn't drawing from below yet, and you're watering only the top layer of soil that's dried out. Some strains handle that better than others. In my experience, temperature also plays a role in how moist the soil can be. Just let them dry out longer or give less water closer to the seedling.