Calibre puts a unique identifier at the end of some folder and file names to ensure that it can store different books having the same title and author. Unfortunately, Google Drive uses numbers at the end of a file name to indicate conflicting versions. Because of this, Google Drive can (and does) change the calibre identifier to some other number, ensuring that calibre can no longer find the book. From calibre's point of view, the books are lost. This is not an academic concern. It has happened to many people's libraries, including our own.
For more information see the thread WARNING - Google Drive Incompatible with Calibre! in the calibre forum.