Answer: macOS stores fonts in a cache for quicker access. Sometimes, this cache may become corrupted. When this happens, apps bought from the Mac App Store may not be able to display all your fonts. Fortunately, there's a quick way to fix this:

1. Quit all running apps.
2. Launch the Terminal app. (You'll find it under Applications/Utilities.)
3. In Terminal, copy-paste or type the following commands to clear the macOS font cache (press Enter after each command):
   atsutil databases -removeUser
   atsutil server -shutdown
   atsutil server -ping
4. Launch Ultra Character Map.
5. Select Empty Cache from the Ultra Character Map menu.

All your fonts should now be available under the Font menu. You can read more about the macOS font cache in this CNET article: Font problems and management in OS X.