Can't sync PDFs from Books to iOS

I just saw a problem today and I’m wondering if others have seen and possibly solved it.

When connecting my phone (and also iPod Touch) to my Mac (Big Sur, with the latest updates) to sync it, none of the books and PDFs I manually added to books will sync. When I connect the device and look at the screen for syncing books, they are not on the list.

If I open the Books app on the Mac, they are visible and I can open them just fine, but they don’t sync. If I actually perform a sync, they get removed from my device, forcing me to manually add them back on the device via a web page where I have them hosted. Other content seems to sync fine.

I already tried rebooting the Mac and checking for additional updates. I left Apple feedback reporting the problem, but since Monterey has been released, I’m sure they won’t care unless the problem is happening there as well (which I don’t know).

Any ideas?

More weirdness. None of these books show up in the sync window in the Finder. But when I went to sync my iPod Touch, they were showing on the device with a cloud icon. Tapping them downloaded them, and I confirmed that they were the latest versions that I just created today.

But this is not happening on my iPhone running the same iOS 15.1.

It would seem that local books are syncing through iCloud, even though I can’t remember ever opting in to such a service, but why would it only be working on the iPod and not on an iPhone?

Is there perhaps a settings I need to toggle somewhere?

This morning, I did some more searching and found that there’s an option in iOS 15 to sync PDFs via iCloud:

When I turned that on on my phone, the books from the Mac appeared (with cloud icons) and could be downloaded that way. Updating the books I manually add (that is non-store books) to my Mac causes updated copies to appear on my devices with explicitly syncing them.

It still strikes me as odd, however, that there no longer seems to be a way to sync PDFs over USB the way I always did in the past.