Leaf issue

Marijuana is an annual plant that only wants to live, breed and die. Think of it like a plant version of Mr. Meseeks... it only wants to live long enough to carry out its function and die. Cannabis produces trichomes to get pollinated and to protect itself from pests during bloom. It’s perfectly natural for fan leaves to die back in flower, especially in later flower. Looking at your diary, I think your air temperature may be getting too high and possibly fluctuating during the day. I know in my tent if the air temp gets above 80 I typically see signs of heat stress. At week 14 I can see a few burnt tips and at week 15 I see additional signs of possibly nute burn looking at the leaf tips and edges. It could be a cal/mag deficiency... but you appear to be already feeding cal/mag. If you’re using tap water, I really don’t think this would be an issue... especially in Oklahoma with all the minerals in the water. Best of luck!👩‍🌾🏼🌱