Three members of our team have the Craft plugin for Sketch. We've verified that we're all using the same version of the plugin and the same version of Sketch. Two of us have Google Drive for macOS, and the third has Backup and Sync (which is the Google Drive replacement now that it's been deprecated).
When I add colors to the Library and my colleague with Google Drive is the only person also using the library, syncing of my new colors works fine. As soon as our third colleague with Backup and Sync joins syncing abilities stop for all of us. And even after that person removes the library, no one can sync any more.
On my local machine Google Drive says everything is in sync, but in the web interface I can see that what is in Finder doesn't match. New color folders inside the library folder are not getting added.
As best we can tell, any time we do anything as a pair (using only the Google Drive client) everything is fine. As soon as one person using Backup and Sync joins, syncing of Craft Libraries stops for everyone. And finally, looking at Google Drive on the web, it looks like the hidden file ./library-index/index is always the last file to change before syncing stops working.