To make it a bit more detailed: This even worked cross-platform (tested with PW3 and KT3, both running fw 5.8.11). Unfortunately, the content of the documents folder isn't exactly the same on both devices.
All my private documents are the same on both Kindles, but the only "truly Amazon" book differs in name and contents: the Users Guide.
This allowed me to check what happens then. Of course, when selecting this entry in the respective collection on the "wrong" Kindle, an error occurs: "This book isn't on the device." Removing this entry from the collection and immediately re-adding it, doesn't help. Same error. I had to completely delete the book from the device and sideload it from a backup after a restart of the Kindle. Then it could be added to the respective collection and was usable again.
Gesendet von meinem Google Pixel mit Yabba-Dabba-Talk.