iPhone app on iPad compatibility mode has strange layout in landscape (while I force portrait)

The Problem

As you can see, my iPhone app has a very strange layout when starting on iPad in landscape mode. I force the orientation of the app to portrait, but only on the iPad in landscape mode (when I start the app) seems to put the view 90 degrees turned in a portrait container. Also, the status bar seems to be a black bar and in the middle of the screen.

The app seems to work properly when I start it from portrait mode.

When starting the app with the iPad in landscape mode

When starting the app with the iPad in portrait mode

My code & configuration

The app has all 4 orientations enabled in the info.plist (because I dynamically switch orientation depending on which viewController is active).

I force the app to be portrait in the first viewController by using:

- (BOOL)shouldAutorotate {
    return YES;
}

- (NSUInteger)supportedInterfaceOrientations {
    return UIInterfaceOrientationMaskPortrait;
}

I tried setting shouldAutorotate to NO, but this makes no difference.

The way I am starting my app in the AppDelegate didFinishLaunchingWithOptions is:

[[UIApplication sharedApplication] setStatusBarHidden:NO];

// 1
UIStoryboard *mainStoryboard = nil;
mainStoryboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
// 2
self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
self.window.rootViewController = [mainStoryboard instantiateInitialViewController];
[self.window makeKeyAndVisible];
if ([self.window respondsToSelector:@selector(tintColor)]) {
    self.window.tintColor = [UIColor whiteColor];
}

Strangely, everything goes 100% right with testing on an iPhone. Only the iPad compatibility mode has this issue.

Question

How can I solve this orientation problem for the iPad compatibility mode? It seems I am missing something that only applies for the iPad compatibility mode, as nothing like this happens for the iPhone.