This happened with the Microsoft ebooks store and has happened with countless other DRM media.
Sometimes books are automatically revised, or censored. Sometimes accounts holding DRM licenses are deleted or deactivated by mistake.
The only way to actually own a book these days is to buy it on paper, or from one of the rare few publishers that do not use DRM.
If you want a DRM free e-book collection you will need a book scanner or send it to a scanning service.