Based on another thread on this forum (
https://www.mobileread.com/forums/showthread.php?t=62701 ), I'd like to recommend that you update your first post to include a best practice for using Calibre + Dropbox:
"Always close Calibre after using it, because otherwise the metadata.db file won't be synced correctly, which will lead to problems if you open Calibre on another computer."
Or something like that :-)