I get this as well.
It's completely random and very occasional.
It's not that much of a problem.
It only stops if/when I open a new Microb page/page from bookmarks

As you say, this 'no button' look appears to be portrait mode stuck in landscape.

Anyway - still don't understand why they even bothered with portriat mode as it doesn't have a back button, which is very annoying as basic web browsing is impossible.