The Photos app's filter-based "smart albums" build their file listing using the search configuration (photosSourceFolders) of the user viewing the album, rather than the album owner's configuration. …

Summary

The Photos app's filter-based "smart albums" build their file listing using the search configuration (photosSourceFolders) of the user viewing the album, rather than the album owner's configuration. When an album owner shares a smart album with another user, that user's own folder configuration is used to determine which of the owner's files are searched — allowing them to discover files (name, file ID, and other metadata) in folders the album owner never intended to include in the shared album…