This may be a Discourse thing and therefore not fixable - but even in an incognito window/tab if you login, and then log out, then want to login again with a different address it just logs you back in as the previous person!
That means any sort of public computer is not safe to use - which might not be a big deal, but security wise it’s a huge flaw.
I even tried opening up a completely separate incognito window - the only way that works is if you have no incognito windows/tabs open anywhere, which is just frustrating as I use incognito for most things on computers that aren’t mine (eg. my work computer).
I have 2 logins cuz the 'Forge was originally bought under a different one and if I need to add users or similar I have to login using the old one.
Chrome Version 69.0.3497.100 (64-bit) running on a Win 10 desktop (though I doubt that matters)