Vapor Pressure Deficit... ok, I've got a new thing to start learning about. You are spot on with our humidity being very low. I am in the Sierra Foothills of Cali, so we are like 20-30% humidity this time of year.
I start my plants inside, where I can control temp and humidity basically perfectly, then I slowly harden them off and move them outside.
This is kinda all starting to add up that the biggest, most indica heavy plant, is having the issues. The new new growth seems to be doing well, since I increased the watering.