Xamarin.Forms on iOS is not showing NavigationBar at the first page

Hello guys,

i am struggleling since a few hours. I do an Xamarin.Forms app targeting UWP and iOS.
On UWP everything works good. But on iOS the first page is not showing up the NavigationBar. I explicitly set

NavigationPage.SetHasNavigationBar(this, true);

I also set it in the xaml. I wrapped the page within a NavigationPage. I navigated through Prism.Forms as well as creating it by hand:

Application.Current.MainPage = new NavigationPage(new MyPage() {Title="bla"}) { Title ="blablabla"};
I'm using the most current Xamarin.Forms Version: 3.1.0.697729. Is there a bug?

best regards,
Chris