I would have said a lack of potassium or magnesium, it seems hard to believe given the soil you're using and the fertilizer you applied recently. As others have said, it might be the pH; yours is between 5.5 and 6.4, so basically the average is 6 and you're watering around 6.3. Knowing that in soil these 2 elements (Mag and K) are better absorbed between 6.5 and 7, you could be hindering their uptake with your pH of 6.3, which could lead to a deficiency of these 2 elements due to the difficulty in absorbing them properly. It makes sense, but I still have some doubts.