Some leaves from the top of my biggest plant, Frisian Dew, turn yellow and brown. The others don't show this behavior. The plant is in week 22 and still needs approx. 4-6 weeks until harvest. The plant pot is fully rooted up to the surface.
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.