Since it looks like the tops of the plant is getting yellow, it might be an immobile nutrient issue rather than a NPK issue. One of the plants there looks like she has a bit much nitrogen but not the one with the discoloration. They're not clawing or curling so I don't think nitrogen is the main culprit. If it was potassium, then the lower leaves would be showing the deficiency not the tops. Potassium is a mobile nutrient. It would have to be an immobile nutrient but it doesn't have spotting like calcium. Sulfur deficiency would make it turn colors, but it's usually red and purple. It could be an iron deficiency. It's only in one area. Maybe it's light stress. Not every cultivar is cool with 24hr lighting. Some might be more finicky because the test is your plants look awesomeðŸ‘
You still have about a month to 1.5 month left of the grow. In about 2 or 3 weeks, your going to want to cut off the nutrients and let her start finishing off and start to fade. If you do decide to add nutrients, just add in small amounts. If you're on week 8 or 9 then, 4 of those was veg, 4 weeks bloom. You only have about 4 to 6 weeks more depending on variety. I gave my most recent Blue dream a feeding too late into flower and it possibly caused her to herm on me. Didn't expect her to finish so soon 😅
Possible causes:
since it's a top, it could be too close to the light and could be light stress.
Genetics
Could be a calcium, iron or sulfur deficiency (not likely)