It's intentional behavior. There isn't much use for a header during input with an on-screen keyboard on a mobile phone. Tablets are a different story, but you can easily change the behavior as you point out above.
The fixed header just takes up valuable screen space which is already quite minimal when the keyboard is shown. I don't understand why anybody would want to fixed header to show while the on-screen keyboard is shown, but you are free to override the default behavior.
Yes, the new documentation is a disappointment. The old documentation was much better, and that's not saying much.