What is the difference in the two tool options:
Update metadata from existing musicbrainz id or release id
and
Auto correct metadata from musicbrainz server?
Difference between Update metadata from existing musicbrainz id and Auto correct metadata from musicbrainz server
The first is used on songs already matched to MB, it just updates the songs with any extra info that has been added to the song within the MB database since you originally matched it.
The second is used to actually match songs that are currently not matched to MB. With default options it wil actively ignore any already matched songs but you can change this in the settings if you want it to rematch songs as if they were not already matched.