I have configured Preview for my website and it was working great when I was using "fallback: true" since if the page didn't exist, it would build it and display the content.
However, I had to switch to "fallback: false" since I had some social sharing issues with "fallback: true" (Facebook/Linkedin didn't fetch the correct meta tags). This means that if a page is not been build, it will return 404.
The problem is that if I have not published a page yet, if I go to preview it, it will return a 404 for me. It is not a problem if I have build the page and then preview, then it works great.