Leaf started yellowing and growing a weird shape. Too much light? Too much water?

If it's an overwatering issue, the check is simple: let the substrate dry out and wait for the seedling to show signs of water stress (be careful, you need to be vigilant since the plant is young). Then, resume the watering schedule divided by 2 (it's up to you to adjust this value if you notice that the watering rhythm becomes too intense). Given the symptoms, I would lean more towards either too much light or an unsuitable pH of the water. Seedlings need little solar energy, and depending on your power, I would test more like 10 or 20%, that should be more than enough. If that's not enough, you'll see your plant stretch, and then you'll just need to dim to a higher value. As for the pH of the water, I know that according to belief, when it's in soil, we don't regulate it, but I tend to believe that young seedlings in a small volume of substrate, the pH of the water is impactful, especially if your water has high values (friends of mine have a pH of 8.8 coming out of the tap). Moreover, a low pH is rather beneficial for young plants (personally, I start at 5.5 and then 6 depending on my seedlings' reactions). There's also the humidity level (I aim for 60% or 70% max) which can sometimes be too high and, depending on the conditions, allow for the development of fungi. Anyway, several leads, you let us know!