books keep deleting from device

My wife and I have identical iPad minis with the same OS. We share an iCloud account. We read a lot of books, but on her device, she can download a book to the iPad (indicated by the small cloud icon below the book name) and see it there, but hours later Books has deleted it from her device and the little cloud icons reappears. This does NOT happen on my device, so I’m guessing a setting we have just missed. This is very frustrating. We went on a plan trip recently, she downloaded the books to read before we left home, but when she got on the plane, those books were once again not downloaded. She has more free space on her device than mine, but this does not happen on my iPad. Anyone have ideas how to stop this behavior?

fry: our iCloud sync setting are exactly the same.