This latest update includes two improvements that could create new metadata, new files, or in some cases, re-import existing files.
Improved Face Recognition - the enhanced face recognition feature can now find faces in photos previously missed or not identified. These new faces will add metadata to your library that can present as new XMP sidecar files. These, in turn, will appear in the Sync Panel as Optimized images requiring synchronization.
Apple Photos Library - Apple has introduced changes that provide a better method to track and synchronize photos originating from an Apple Photos library. To take advantage of these changes, any linked Apple Photos Library must be re-scanned. Depending on the library's state, this may result in re-imported or duplicate images. You should be able to remove any duplicates generated by this process using the Photo DeDupe Tool.