I bought a book from Amazon that did the same thing. It would stay in the collection a few days and then not be in it anymore. I'd put it back and that would repeat. I finally read the book and removed it to solve the problem.
It seems I might have had that problem with another book some time a year or two ago but I'm not sure I remember it right. I don't remember any details about that one.
I just assumed this was some glitch somewhere in the system. It's not a common problem for me so I don't worry about it.
Barry