Book Title does not appear in RAB if using Thai Script

I’m working through an example app trying to bring myself up to spead before I demo RAB to another group. That language group uses Thai Script, and has a preference for the Sarabun font.

RAB does not seem to be able to present Thai Script in the app builder. The app works fine, and you can view the text in the browser without issue.
In our list of books we see this
I’;ve added the Sarabun font to RAB and have set the book to use Sarabun.
The text in the viewer looks like this

The text in the book file looks the same…tofu boxes.
How do I get RAB to present the Thai script properly in RAB itself? Again the app works fine.
Thank you.