Specifying which orientations the iOS app supports are not honoured correctly. ie: a landscape only game will still render in Portrait mode even when Portrait is disabled in the iOS Settings panel
1) Under iOS Project settings, disable "Supports Portrait Orientation"
2) Create a shipping build and install onto iOS device
3) Build should not be able to run in portrait mode
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-198625 in the post.