When UIKit detects a change in device orientation, it uses the `UIApplication` object and the root view controller to determine whether the new orientation is allowed. If both objects agree that the new orientation is supported, then auto-rotation occurs. Otherwise, the orientation change is ignored.
By default, the `UIApplication` object sources its supported interface orientations from the values specified for the `UISupportedInterfaceOrientations` key in the applications' Information Property List. You can override this behavior by implementing the `application:supportedInterfaceOrientationsForWindow:` method in your application's delegate. The supported orientation values returned by this method only take effect after the application has finished launching. You can, therefore, use this method to support a different set of orientations after launch.
Allowing your app to rotate into portrait after launch.
- (NSUInteger)application:(UIApplication *)application supportedInterfaceOrientationsForWindow:(UIWindow *)window { return UIInterfaceOrientationMaskAllButUpsideDown; }
source: [https://developer.apple.com/][1]

[1]: https://developer.apple.com/library/archive/technotes/tn2244/_index.html#//apple_ref/doc/uid/DTS40009012-CH1-TNTAG5

I had the same issue when launching our app in landscape on an iPhone 6 Plus.
Our fix was to remove landscape supported interface orientations from the plist via project settings:
![Removed landscape orientation][1]

[1]: http://i.stack.imgur.com/HY1pl.png
and implement application:supportedInterfaceOrientationsForWindow: in the app delegate:
- (NSUInteger)application:(UIApplication *)application supportedInterfaceOrientationsForWindow:(UIWindow *)window { return UIInterfaceOrientationMaskAllButUpsideDown; }
Apparently the information in your plist is to specify what orientations your app is allowed to launch to.