Usually when it progresses around the serated tip, it's K-deficiency.
22 weeks? how large is this pot? you may have root-related issues if the roots are congested. Can pop it out and look - if there;s a lot of looping and overlapping around the outside, that's a good chance it is rootbound. Would see more than 1 issue in this case as it would struggle with nutrient uptake of more than one molecule.