PDF files in Apple Books are not syncing between devices

For some strange reason, PDF files I drop in Books on any of my devices are not showing up in Books on my other devices.

I have three devices:

1. MacBook Air M1 2020, macOS Sonoma 14.5
2. iPad Pro, iPadOS 17.5.1
3. iPhone 14 Pro Max, iOS 17.5.1

On each device:

• I’m logged in with the same Apple Account for the device itself, iCloud, and Apple Books.
• “Books” is turned on in “Settings > [My Name] > iCloud > Apps Using iCloud.”

The weirdest part of all this, is if I drop PDF files in Books on macOS, I see them in this folder:

~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books

Yet I see all the other PDF files uploaded from other devices to iCloud here:

~/Library/Mobile Documents/iCloud~com~apple~iBooks/Documents

So I guess the PDF files are getting synced, at least in one direction, but why is Books on macOS too stubborn to use iCloud like it’s supposed to?