This might be a dumb question–but how do you view it in incognito mode? The situation where it does not work is only inside the v0 app area. Once my app is published, it no longer has the issue (which is unfortunate, I guess, because that would be how I know how to test in incognito mode).
BTW I was never able to figure this out but it definitely appears to be an issue with the preview environment not reflecting reality. Sadly, because of this and intermittent support for firebase authentication, I’ve pivoted to no longer using the preview environment at all, and instead just publishing directly all the time (this is my new preview environment), then having a separate production environment.