Hmmm, that is strange. It is just a link, so if you're already logged in in that browser (for the roadmap), you should stay logged in.

Beyond the initial "is this user Plus? If so grant them access", we don't control the authentication and cookies for the roadmap itself, which is run by Noora. However, you should still say signed in. I've just tried both from the landing page and app website and I was logged in.

Which makes me wonder: is the roadmap opening in your mobile browser, which doesn't store cookies or is in private browsing mode, perhaps?