Newly Imported Books Are Not Being Uploaded to iCloud on Mac

Imported books (PDF/EPUB) are not uploading to iCloud after I successfully imported them into the Apple Books app on Mac. However, if I send them to my phone (email/airdrop), upload them there, voila, they are uploaded perfectly to iCloud and IMMEDIATELY appear in Books on Mac, meaning the Mac downloads them perfectly from iCloud but does not upload them itself. I tried basic operations with iCloud on the Mac, such as Authorize This Computer (Books -> Account -> Authorization), and checked in the settings that the Books app is enabled for iCloud synchronization (Settings -> Apple ID -> iCloud -> Apps Using iCloud -> iCloud Drive -> Apps Syncing To iCloud Drive -> Books is enabled). Nothing helps. I want to upload my books on the Mac and then read them on my phone; please help me restore this function (it used to work fine before). Thank you.