I, too, have this problem, except I don’t do a hard reboot. All I have to do is wait about 10-15 seconds after my touchscreen either freezes or, more often, begins to react erratically to touch. Occasionally it’ll restart with no warning at all. It’ll restart itself, taking me to the black screen with the little circle of rotating white lines...then to the pin screen. As soon as I log back in, all of my previously active screens are still there. My restarts are not always associated with Safari use. Sometimes I am only using apps and haven’t even touched Safari. My iPad is 1 1/2 years old and just started this about 2 weeks ago, as many as 5 times in a day. I’m running iOS 11.3.1. This is intensely annoying and sometimes costly when I’m doing something where seconds matter. What gives?