What do you think the problem could be?

Kind of looks like a potassium deficiency to me. Based on it having more “blotchy” looking markings of yellow opposed to if it were an Iron deficit the whole leaf would be turning yellow. Potassium also starts in the newer growth as well so that’s kind of the only thing that resembles the same markings in your plant. A pure potassium plant solution would help as long as you follow the directions. That way you’re not having to over do your other nutrients. You can also use potassium as a foiler spray and plants love it. Mr. Grow It uses a potassium spray during veg on all his grows.

I don't think it's a nitrogen deficiency. Because their problem starts from the top of the plant. And nitrogen deficiency starts from the bottom. I've done a bit of research and concluded that it might be an iron deficiency. Iron deficiency is usually due to a pH (alkaline) that's too high, and it can be corrected by flushing the medium with water at the correct pH, and if necessary, an iron supplement can be added. Iron can react with other nutrients and cause its own problems, so it's important to proceed with caution and observe all changes very carefully.