I took a peek, and I noticed that, when I preview the Home page, there is a page transition in which everything moves from the top down, including the navigation bar. If you select Body in the Navigator panel, and then view the interactions in the Object panel, it shows that you have an interaction applied to the entire body of the page.
It’s possible that, because you have this interaction applied to the entire body of the page, it’s overriding the fixed positioning on the navigation bar.
As a test, I changed the Body interaction to None, and then the navigation bar was successfully fixed in the Preview.