Agreed - it's an annoying little bug that we're going to fix. I am not sure what is going on, but I think I've nailed down when it happens: If you log into the site, and then close your browser completely without logging out, and then try to launch a post directly from a link in a notification, you'll get bounced to the homepage. You can click the notification link a second time, and you'll get to the correct post just fine, without having to sign in at the homepage.
This behavior only happens when you close your browser without signing out. If you do sign out of the site before closing your browser, you won't have the issue. Similarly, if you browse the site when NOT signed in, then close the browser, then click a link in a notification, you'll again reach the correct post right away.