Audiobook chapters not appearing in Apple Books on iPad Pro

I have some m4b audiobook files that I have successfully put into Apple Books on my iMac (OSX Ventura 13.7.8) and synced to my iPhone 14 Pro Max (iOS 26.2). The books on both of these devices are fine and both split the audio book successfully into Chapters.

However, when the books are synced onto my iPad Pro (12.9" 4th Gen - ipadOS 26.3) the books are NOT split into chapters and it just loaded as 1 long audio track.

The Books app is Up-to-date onall 3 devices.

Any ideas why the books are fine on the iMac and iPhone.. but NOT the iPad?

Thanks