Audiobook Last read position not sync’ing across devices

I have some audiobooks stored in Apple Books on both my iPad and iPhone. Some of these books are aax (audible) files and some are just mp3 files.

when listening to the aax files, the last read position is getting sync’d across devices. So I can listen to a book on my iPad, and when I listen to it on my iPhone it will pick up from where I left off.

however this isn’t working for the mp3 files. These files remember where I left them on the individual device, but they are not being sync’d across. So I can listen to a book on my iPad, but when I go to my iPhone it starts from the beginning.

I have iCloud Drive set on, and Books set on in iCloud settings on both devices.

Any ideas how they can be be sync’d across my devices