Currently, books that are prioritized but not downloaded are hidden from the Prioritization tool on Desktop, even though they can impact prioritization. This can cause a lot of confusion. But starting from v35, cloud books, Print Library books, and other unowned books will now be visible in the Prioritize Books pane with an icon to indicate their special status. These books can now be added, removed, and rearranged as normal. (It's not possible to add other unowned books to your priority list, but if you've previously added one and later lost the license, the unlicensed book will now be visible.)
Because this data was previously hidden but not deleted, don't be surprised if you find duplicates or other long-forgotten entries that are now visible in the list. It's possible some previously hidden entries may also have been moved to the end of the list.