When did books stop syncing?

I have all my books stored locally on my Mac and added to the Mac Books app. Historically, they've synced during normal syncs that I perform with my iPad or iPhone plugged into my Mac.

But this apparently changed at some point. Today I tried to access a book in my iPad, only to find that none of the books in my library existed on my iPad anymore. The Books app wanted me to download every book, one at a time, from the Internet... which I had no access to.

With the iPad plugged in and pulled up in Finder, there are no syncing options for Books. There's no heading for it in the UI, and Books is not listed under Files. Does anybody have an explanation?