Some epub books not syncing to iCloud

I am experiencing a weird scenario where I have a batch of 13 epub books which are not able to sync to iCloud after adding them into iBooks.

My iBooks currently has several epubs and pdfs and they are syncing fine to iCloud.

For test sake, I downloaded another new random epub book, added it to iBooks, it appears on my iPhone's iBooks within seconds. Then I tried to delete it from my Mac, it will prompt that it will be deleted from iCloud and all devices.

Next, I tried to add one of the affected epub book into iBooks on my Mac again. I can open and read it fine but it just doesn't get uploaded to iCloud. When I tried to delete it, it will only prompt about being deleted from mac.

I discovered these affected batch of 13 epub books when I turned off syncing iBooks in my iCloud preferences.

Any advise will be greatly appreciated!