iCloud keychain backup/restore behaving differently on iOS 11?

Question

If a backup to iCloud is made from one device then restored onto another device then items stored in the keychain aren't restored. This isn't what was expected, AFAIA a "ThisDeviceOnly" access setting isn't used, kSecAttrAccessibleAfterFirstUnlock is used (as opposed to kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly) which should have resulted in items being restored onto the other device?

class func createKey() -> NSData?
{
    let keychainIdentifierData = kKeychainIdentifier.data(using: String.Encoding.utf8, allowLossyConversion: false)!

    let keyData = NSMutableData(length: 64)!
    let result = SecRandomCopyBytes(kSecRandomDefault, 64, keyData.mutableBytes.bindMemory(to: UInt8.self, capacity: 64))
    if (result != 0)
    {
        return nil
    }

    // Store the key in the keychain
    let query: [NSString: AnyObject]  = [
        kSecClass: kSecClassKey,
        kSecAttrApplicationTag: keychainIdentifierData as AnyObject,
        kSecAttrKeySizeInBits: 512 as AnyObject,
        kSecValueData: keyData,
        kSecAttrAccessible: kSecAttrAccessibleAfterFirstUnlock
    ]

    let status = SecItemAdd(query as CFDictionary, nil)
    if (status != errSecSuccess)
    {
        return nil
    }
    return keyData
}



    class func getKey() -> NSData?
    {
        let keychainIdentifierData = kKeychainIdentifier.data(using: String.Encoding.utf8, allowLossyConversion: false)!

        // First check in the keychain for an existing key
        let query: [NSString: AnyObject] = [
            kSecClass: kSecClassKey,
            kSecAttrApplicationTag: keychainIdentifierData as AnyObject,
            kSecAttrKeySizeInBits: 512 as AnyObject,
            kSecReturnData: true as AnyObject
        ]

        // To avoid Swift optimization bug, should use withUnsafeMutablePointer() function to retrieve the keychain item
        // See also: http://stackoverflow.com/questions/24145838/querying-ios-keychain-using-swift/27721328#27721328
        var dataTypeRef: AnyObject?
        let status = withUnsafeMutablePointer(to: &dataTypeRef) { SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0)) }
        if status == errSecSuccess {
            return (dataTypeRef as! NSData)
        }
        return nil
    }