You are probably seeing a session issue because the session is serialized as a cookie, and that cookie will only be read after a page render. This is referenced in “It’s important to understand that Session and Flash data are not stored in the server but are added to each subsequent HTTP Request, using Cookies.”
The cookie value on the request is immutable and you’ll only see the changed value on subsequent requests. If you send a Redirect(url).addingToSession(key, value) and then the subsequent get will have the session after that.