Apple books uses iCloud in all my devices, however I only see some of the books in one of my devices

Over the years I have grown a huge Apple Books library, bringing there my books from various sources.

I use Apple books on different devices (iMac, MackBook, iPad, iPhone).

Until some time ago, I could see all my books in all my devices.

Since a few months I noticed that this was not the case anymore.

I usually add books to Apple Books on my MacBook, as it's easier to manage libraries, rename books, etc.

So what happens lately is I add books on my MacBook, they get added to iCloud, and then I can also see them on my iPad.

If I close and re-open Apple Books on my MacBook, some (or all) of the newly added books are missing.

I can still see them on my iPad, however they are missing in Apple Books on my MacBook, and similarly missing on Apple Books on my iPhone.

I tried turning iCloud off and on, adding books again (which did not work then, as they probably already are in my library), nothing.

Any ideas what could be the cause, and how I can fix it?

I am really worried I will loose books at some point.

Thanks in advance