Audiobooks not syncing via iCloud
I'm pretty sure this used to work, but at some point my audiobooks (not bought through Apple) stopped syncing between devices via iCloud. All devices have iCloud drive enabled and iCloud enabled for Books. I can verify that because my PDFs do sync via iCloud. It's only audiobooks (like from Audible) that will not sync via iCloud. If I turn it off, the PDFs disappear.
I have two computers running macOS 12.6 and one iPhone with iOS 16 (although this didn't work in iOS 15).
Mac mini 2018 or later