I'm trying to understand either it's a bug or it's the expected behavior.
On iOS 10 and earlier we could set up a custom title, using navigationItem.titleView.
On iOS 11, when setting our navigationItem.largeTitleDisplayMode = .always and setting navigationItem.titleView = <Some cool UIButton> it's displaying both the normal navigation title bar and the large navigation title.
To sum up:
How can we use custom titleView on our Large Navigation Title?
EDIT : This is the expected result:

