I suspect that the system never really thought through that far; so it is a bug, but nobody would 'expect' anything.
Try leaving ADE, editing the file mainfest.xml (probably in your My Digital Editions directory) and removing any sections that reference the book:
remove everything from and including <de:contentRecord> to and including </de:contentRecord> for the book.
... Oh, and delete any copies of the .epub file as well before you retry.
Then restart ADE and try the .acsm file for the second library again.