Chapters in my m4b audiobooks have disappeared

All the audiobooks that I copy to my iPhone lose their chapters when playing them back in the Apple Books app. The Books app on my Macbook Pro sees them just fine, showing chapters and allowing you to select between them. This is a recent occurrence, as the chapters used to show in the iOS Books app during playback, and you could select between them just as in the macOS app. Now each audiobook is just shown as one long book with no chapter markers even though they are present within the file.

Audiobooks downloaded from the Apple store seem to not be affected; only M4B audiobooks that I have ripped myself. The files themselves are fine. Playing them through the Audible app on the iPhone shows the chapters. It is only the Apple Books app itself that has the issue.

I have restarted the iPhone but the problem persists.