Hey, We are aware of an issue with Letterboxing (black bars) impacting the sides on iOS devices after the 10.31 update. A potential workaround is uninstalling and reinstalling the Fortnite application, this may fix this issue. You can follow along with the Trello card here.
https://trello.com/c/tUnhQNIl/414-letterboxing-occurs-on-ios-devices