UICollectionView in landscape on iPhone X

Having the same issue. This worked for me:

override func viewDidLoad() {
    if #available(iOS 11.0, *) {
        collectionView?.contentInsetAdjustmentBehavior = .always
    }
}

The documentation for the .always enum case says:

Always include the safe area insets in the content adjustment.

This solution works correctly also in the case the phone is rotated.