This was happening to me on my fully updated 6s, and it's been happening since I got my phone but I kept brushing it off. Well today I decided I'd finally try to figure it out and I either got lucky or fell into a little trick. Like most people everything but calculator worked (why I've waited so long to do anything about it) and none of the hard reboots or setting resets worked. So with my phone locked (and I do use the password) I opened camera, then opened calculator and it worked! I then let it go black, turned it on to the lock screen again and opened calculator and it stayed open! Hope that helps.