AdditionalParameters in response are always null

[benmac]

I do not know if this is a bug or a lack of understanding on my part. We are using the library with Identity Server 4. and attempting to make use of the additionalParameters on the response. The reason for this is we have a language selector on the IDS4 login page and we wish to send the language back to the app, so the app can adjust accordingly. Identity Server has been modified to send the token back to the app with a return url of OurAppName:/oathredirect?lang=en.
Using Charles (Web debugging proxy) we can see that the response is definitely sent with the "lang" url parameter(see attached land.png).

Code to call IDS4

return authorize(Auth.getConfig())
      .then(authDetails => {
        const { lang } = authDetails.additionalParameters;

        if (!lang) return authDetails;

        return Auth.setUserLang(lang, authDetails.accessToken).then(() => authDetails);
      })
      .then(authDetails => {
        store.dispatch(addAuth(authDetails));
        return authDetails;
      })
      .catch(e =>
        Auth.logout().then(() => {
          throw e;
        })
      );

static getConfig() {
    const { apis: { identityServerURL } } = store.getState();

    return {
      issuer: identityServerURL,
      clientId: IDENTITY_SERVER_CLIENT_ID,
      redirectUrl,
      additionalParameters: {},
      scopes: ['openid', 'profile', 'offline_access'],
      dangerouslyAllowInsecureHttpRequests: true,
    };
  }

Yet when run const { lang } = authDetails.additionalParameters;

authDetails.additionalParameters is always null

Am i missing something in how i am doing this or is this an issue?
note: i have tried with additionalParameters: {}, in the config and without

[kadikraman]

additionalParameters is now deprecated. It wasn't implemented consistently across platforms and was problematic because out authorize call does two api requests behind the scenes, and both may carry additional params.

To account for this, we've updates the authorization result returns two parameters:

authorizeAdditionalParameters - the additional params from the /authorize request
tokenAdditionalParameters - the additional params from the /token request