Preview not showing all fonts that are installed and showing in the Font Book

Can you give an example? Do you have a specific font in mind? There are hundreds of fonts available in the system. (See https://support.apple.com/HT213266 - the server is broken right now). Most of those fonts are intended for document support. That means that is you have a document that already includes them, then they will be available. Otherwise, you will not be able to use them.

This feature, and many other font-related features, will be broken in most 3rd party apps. This is one area where 3rd party developers, including those from major design companies, rise to new levels of incompetence. Pages should work properly.

Preview, however, is an odd duck. It really isn’t a document-creation tool. It may not support all font features. Plus, Apple is a big company. Some Apple developers, like those that work on Pages or Keynote, are better than most. But others, including anyone assigned to work on Preview, are not.