I believe this is just a minor issue.
If in MusicBrainz Settings the option “Do not match online if already have a MusicBrainzID” is active, then the “Manual correct from MusicBrainz” comes up empty on a track that does indeed have an ID.
Since the user has selected that option, the “manual correct” panel should not appear at all or the command should be unavailable. Another possibility would be to allow manual correction as a kind of settings override. The way things are now, that empty panel showing up can be confusing because it gives the impression that no matches were found.