iOS 11 prefersLargeTitles not updating until scroll

I recently hit the same issue and none of the suggestions worked for me. Instead, all I needed to do was to invoke sizeToFit(). Sample code:

private func configureNavigator() {
    guard let navigationController = navigationController else { return }
    navigationController.navigationBar.prefersLargeTitles = true
    navigationItem.largeTitleDisplayMode = .automatic
    navigationController.navigationBar.sizeToFit()
}

I hope this helps!