@Andy, thank you for sending you project. I found the problem. There was an extra font listed in the font choices for the book collection or book that wasn’t in the list of fonts for the app (it probably got deleted at some point). To me, this seems to be a bug in the SAB desktop app that doesn’t clean these up when a font is deleted.
The native app ignores these entries so I changes the PWA to do the same. If you use the latest PWA source code (see Using latest PWA source code), then your app should work (it did for me).
Chris