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

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.