I'm getting this frequently too. Only a reboot brings the dock back, but it disappears again within an hour of use
I don't believe it's related to any app, since an app should not have the ability to interfere with the OS.
Personally I think it's a bug relating to orientation lock, which Apple have not bothered to test properly in IOS 11.
When the dock disappears I can rotate my iPad into portait mode, and I do see the dock reappear although in completely the wrong position. Going back to Landscape mode the dock is not visible again, so it's like its displaying it outside the bounds of the display.
I have orientation lock almost permanently enabled, which has never caused a problem in previous iOS
Apple Tech Support suggested I reset my Home screen (resulting in all dock folders being lost, and 100+ apps all just reordered randomly). This had zero effect on the dock problem, but it kept me busy for a couple of hours sorting it all back out, thanks Apple!