Landscape app goes portrait during app store login

I'm working on a landscape app and I've encountered a graphical problem during app store login.

On some devices/iOS versions (iPhone 5s running iOS 7.0) when the user is prompted to login to the App Store the app in the background displays improperly. The landscape view is shown on half the screen, and a portrait view on the other half. Obviously this looks very visually unappealing, and I would like to correct it.

I've verified that only Landscape Left and Landscape Right are allowed orientations (both in the Unity Editor and in XCode), I'm using Unity 5.1.2f1 and XCode 6.4.

I've verified that the issue does not occur in iOS 6 or iOS 8.

Any assistance would be greatly appreciated.