Swiss Dream Rosé Auto (Kannabia Seeds)

Photos at the day 49. Overall plant is pretty mature for 49 days. Grow is going good. But I always suffer from one thing, that about week 6-7, top leaves are getting crispy. Even when I use proper PH, Sensi Cal-Mag and all line of Mills according table. Also temperature and humidity is right. I am adding nutrients to demineralized water so it is not possible, that water contains some unwanted parts. This happened with Green House Feeding Short Flowering and also with Mills. It happened in three different grows. My latest theory is, that it is caused by overfeeding. But tips of leaves are burned just very little. Can anybody tell me some reason of this crispy leaves on top bud?