Ah, ok. I always de-DRM everything automatically (opening books in Calibre does the de-drming). It sometimes happen with the newer systems that newer Kindle books can't be de-drmed, but then I return those immediately.

But I'm surprised a book with DRM can even be added to another device than the one it belongs to, let alone be opened?

  > I'm surprised a book with DRM can even be added to another device than the one it belongs to, let alone be opened?

It is probably a path that could potentially lead to another sale. The user reads a few pages of the book, gets hooked, and then is told about the restriction.

That happens on sideloaded books that belong to the same device as well, at least on my Kindle Scribe. These books disappear occasionally, and I have not found a consistent way to make them reappear in the UI. Sometimes they return on their own, other times they do not. Interestingly, their files remain on the Kindle’s file system.