I then did some extensive editing and corrections in the genre field and updated the library from the tags.
Do you mean the other way around? Update Tags (From Library).
This should work, certainly for the [Genre] tag, even without forcing the tag writing with the Update Tags command. In-file tag changes can't commit if the file in question is locked (by another process or by MC itself due to playback), of course, but otherwise, they "just happen".
Are you sure the second copy of MC was actually a "clean" copy with a fresh Library?
Keep in mind, even when you "delete" a file from MC, it continues to "remember" it via the Deleted Items database. When it re-imports the same file, it "reactivates" the old database record, and doesn't "reimport" it.
Of course, even then, AutoImport should pick up tagging changes if the Update for external changes option is enabled in the Auto-Import options section.