It is the default. It is optional though, perhaps you unchecked this option somehow?
Options -> General -> Importing & Tagging -> Update tags when file info changes
Otherwise, it should have, unless it couldn't write to the FLAC files for some reason. Could permissions issues on disk have been the cause of your "tagging problems" you were seeing before? Even then, though, it should have said that it was still updating files down on the bottom status bar.
Did you close and reopen MC before you cleared the Library? Or create a Library Backup manually?
Restoring a Library Backup is the only way to undo major Library changes. MC does make automatic backups at launch, so if you closed/reopened MC before you cleared the Library, then it might have made a backup. The default backup location is in your Documents folder, but you can check the current one here:
Options -> File Locations -> Program Files -> Library backups.
If there is a backup from after you did the tagging, but before you cleared the library, then just restore the backup.