Light burn or nutrient

when the tips crisp like that while yellowing that is a burn from too much which can lead to lockouts so the best cause of action is to do a flush and then start feeding at half what you did when you burnt them

They become very sensitive to nitrogen at this point so its possible they have an N toxicity. With potassium deficiency, you would likely see effects on the lower leaves. What's more likely is that you have micro nute deficiency (like zinc, sulfur, etc.). The most common culprit for that is actually potassium/phosphorus toxicity. PK toxicity actually locks out micronutrients and is my first guess for the issue you're having.