PlayFab Flow

void Start(){
        if (GameCenter.isConnected)
            LoginToPlayFabWithGameCenter ();
        else
            LoginToPlayFabWithDevice ();
    }

    void LoginToPlayFabWithGameCenter(){
        //When there is absolutely no account created on PlayFab, this works as expected: A new Account is created linked to the GameCenter.id
        //Now, when there is already an account created linked to the device ID only, a new account won't be created. Instead, PlayFabId returns that account ID; as if they were linked - but they're not
        PlayFabClientAPI.LoginWithGameCenter (
            CreateAccount = true,
            TitleId = PlayFabSettings.TitleId,
            PlayerId = GameCenter.id,
            OnLoginWithGameCenterSuccess, OnLoginWithGameCenterError
        );
    }

    void OnLoginWithGameCenterSuccess(LoginResult result){
        Debug.Log (result.PlayFabId);
        LinkDeviceID ();
    }

    void LinkDeviceID(){
        //This returns successfully, but the link between this device and the GameCenter account is not shown in the Game Manager
        PlayFabClientAPI.LinkIOSDeviceID(
            DeviceId = SystemInfo.deviceUniqueIdentifier,
            DeviceModel = SystemInfo.deviceModel,
            OS = SystemInfo.operatingSystem,
            ForceLink = true,
            OnLinkDeviceIdSuccessful, OnLinkDeviceIdError
        );
    }

    void LoginToPlayFabWithDevice(){
        //Exactly the same behaviour as with LoginToPlayFabWithGameCenter
        //When there is absolutely no account created on PlayFab, this works as expected: A new Account is created linked to the DeviceId
        //Now, when there is already an account created linked to the GameCenter only, a new account won't be created. Instead, PlayFabId returns that account ID; as if they were linked - but they're not
        PlayFabClientAPI.LoginWithIOSDeviceID(
            CreateAccount = true,
            TitleId = PlayFabSettings.TitleId,
            DeviceId = SystemInfo.deviceUniqueIdentifier,
            DeviceModel = SystemInfo.deviceModel,
            OS = SystemInfo.operatingSystem,
            OnLoginWithDeviceIdSuccess, OnLoginWithDeviceIDError
        );
    }

    void OnLoginWithDeviceIdSuccess(LoginResult result){
        Debug.Log (result.PlayFabId);
        StartCoroutine (LoginToGameCenter ());
    }

    IEnumerator LoginToGameCenter(){
        while (true) {
            if (GameCenter.isConnected) {
                LoginToPlayFabWithGameCenter2 ();
                break;
            }
            WaitForSeconds (x);
        }
    }

    //=====Code repeated just to ilustrate the different paths=====

    void LoginToPlayFabWithGameCenter2(){
        //With CreateAccount being false and no account existing linked to GameCenter, this should return an error. But it does not
        //Instead, it logs me in to the same account I am already logged in: The one linked to the device ID only
        PlayFabClientAPI.LoginWithGameCenter (
            CreateAccount = false,
            TitleId = PlayFabSettings.TitleId,
            PlayerId = GameCenter.id,
            OnLoginWithGameCenterSuccess2, OnLoginWithGameCenterError
        );
    }

    void OnLoginWithGameCenterSuccess2(LoginResult result){
        Debug.Log (result.PlayFabId);
        LinkDeviceID2 ();
    }

    void LinkDeviceID2(){
        //This call would only make sense if an actual account had been created in the first place, but it was not.
        //That is why I try to link this GameCenter account to the deviceID account I'm logged in
        PlayFabClientAPI.LinkIOSDeviceID(
            DeviceId = SystemInfo.deviceUniqueIdentifier,
            DeviceModel = SystemInfo.deviceModel,
            OS = SystemInfo.operatingSystem,
            ForceLink = true,
            OnLinkDeviceIdSuccessful, OnLinkDeviceIdError
        );
        LinkGameCenterID ();
    }

    void LinkGameCenterID(){
        //Again, this returns successfully, but the link is not shown in the Game Manager
        PlayFabClientAPI.LinkGameCenterAccount (
            GameCenterId = GameCenter.id
        , OnLinkGameCenterSuccessful, OnLinkGameServiceError);
    }