iBook deletes the open file

I have experienced a few times that the latest file I have opened - and that remains open - is removed from iCloud. Actually this time two different files (see below). The other files seem to be intact. I was using iBooks from my mobile devices and iMac. It may be that the files that were removed were open at my iPad Pro and iMac (that is two separate files). When I tried to go to iBook on my iPad the app froze. The same happened with my iPhone. After I closed and opened the app, the files were gone. I have had this happened before but I have never been able to identify any pattern nor specific reason for this. For a year ago I bought an eBook and kept it open on my iPad (I don't remember if I also used it from iMac). I lost the eBook and have never been able to get it back. Tried to look on my Time Machine without any success (due to changes of file structure and operating systems?).

I find on the net that other people have lost all their files. In my case it seems to be only the latest file I have opened is lost. Could this behaviour be related to iCloud backup? I am using iCloud backup on my mobile devices but Time Machine on my iMac. I have the latest versions of software.