I think I agree with PeterT, this looks like a continuation of the other problem. The extra info is really just answering some of the questions I posted. But, you are still saying that each time you add a book, one on the device disappears.
If you have books that are going onto the device and then not added to the devices library, that usually means there is a bad book amongst them. Have a look at https://www.mobileread.com/forums/sho...d.php?t=207931
for info about this.
As Geoff said, the unknown author or titles usually mean the metadata is missing in the books. But, some formats don't include it at all. Have a look https://www.mobileread.com/forums/sho....php?p=2455363
for a discussion of this.
What you have not described in any of these threads is what you are actually seeing. I have assumed you are saying that when you send a book, disconnect the device, see the processing screen on the device and then look at the library list on the device, that the new book is there, but a book that was previously in the library list is missing. Can you confirm that is what you mean or explain in detail what is happening?