Here's what happened to me.
I must admit my iPad Air 2 had problems on iOS10 as well, so I'm not sure this is an iOS11 thing.
Also, my "frozen dock" you see in the image could have been caused by the app Linea: it switches to landscape layout automatically and perhaps sometimes fails to affect the dock. Wasn't able to reproduce it though.
happens on my 10.5” pro sometimes too. reminds me of how the iPhone 6 always was. i’d unlock it in portrait and it would get stuck in landscape and finally rotate after a few lock/unlocks and vigorous shaking.