What's wrong with the leaf?

Not a single metric to go on, I can see a hygrometer in a pic what's the temp and humidity at least? Being so young there should not be such problems. What's soil composition, environmentals? Are you feeding yet? Tugor pressure exists meaning water is flowing. Only really 3 minerals that can screw with chloryphyll fluorescence at such a young age. Nitrogen sulfur or magnesium. Magnesium is too distinctive. Sulfur deficiency in plants manifests as pale green or yellowing of younger leaves, stunted growth, and potentially reduced yields. It can be confused with nitrogen deficiency, but sulfur deficiency symptoms are typically more pronounced in younger leaves, while nitrogen deficiency symptoms often appear in older leaves first. Hard to tell with such a small plant to see where starts from. At least from those pics. Not sure tbh, need a little more, but hopefully you should be able to get a idea. Ph, make sure she is in range to uptake. With that out the way it should be as simple as feeding what she needs. There should be a overnight response after feeding. Good luck.

Have you been spraying the plant or wetting the leaves?? New growth is ok, don't worry too much, it is not a disease. Quite often, damage like this is from the leaves resting on the soil at night when the lights are out and the plant lowers its leaves. It is 100% NOT septoria, which is almost impossible to get, unless you dig up soil from an infected tobacco farm and spread it around your pots. Always wait for multiple answers when you post a question and never go with the first random answer.........

Looks to me like a bit of nutrient burn or PH drift on a little seedling. Its not happening on the new growth and that's good, it has stopped in its progression. I would like to think the worst is past and all is fine but monitor for the next week or so to be sure. I do think it is done doing w/e it was, and the old leaves will not repair but the plant will grow.