I'm having carbon copy the same thing happening to me and I first noticed it (initially) happening more after a watering. My lighting is same here as well, 24 inches above the canopy. Adding Cal Mag seemed to help the severity of it. I can only think of a couple things that are causing this still, a few already mentioned here. Heat and wind. the other is from topping or LST. (low stress training). I have two small 6 inches fans blowing down from above, 5 feet away, 24/7, at a low setting but you wonder if that's overkill with the serrated leaves happening. Might have to invest in one of them fancy digital laser infrared thermometer guns to find out the surface temps of them leaves are. Anyone here know what that idea temp should be??