It could be heat related, but I can only guess. I simply water every day to combat heat-stress. None of my plants are showing any signs of stress. I am a little bit like Gigi. I remove older leaves when they begin to yellow. It sure wouldn't hurt to water them with Mg either. I add about 1 tbsp. per gallon. If you are not already doing so, try to keep the soil evenly moist. That's tough to do in these high temperatures, but that's my goal.
I am taking a lot of cuttings right now, and have a couple dozen sitting in the greenhouse. I haven't yet removed the leaves and I can tell you, from seeing all those lush, green leaves on the cuttings, they are completely healthy looking. That's not to brag, but that's just what I see. These cuttings were taken yesterday, so right now, though there are fans running, the temperature in this particularly greenhouse is 120 F.
I think I will bag up all of these. It appears to me that bagging is producing quicker rooting. I had (2) Vera Cruz Rose and a Celadine root in 12 days, and I don't remember ever having any of my cuttings root that quickly.