UIModalPresentationPopover for iPhone 6 Plus in landscape doesn't display popover

Implement the new adaptivePresentationStyleForPresentationController:traitCollection: method of UIAdaptivePresentationControllerDelegate:

- (UIModalPresentationStyle)adaptivePresentationStyleForPresentationController:(UIPresentationController *)controller traitCollection:(UITraitCollection *)traitCollection {
    // This method is called in iOS 8.3 or later regardless of trait collection, in which case use the original presentation style (UIModalPresentationNone signals no adaptation)
    return UIModalPresentationNone;
}

UIModalPresentationNone tells the presentation controller to use the original presentation style which in your case will display a popover.

In Swift 3, if you implemented the original adaptivePresentationStyle method, simply adding this code works:

func adaptivePresentationStyle(for controller: UIPresentationController, traitCollection: UITraitCollection) -> UIModalPresentationStyle {
    return adaptivePresentationStyle(for: controller)
}