https://community.metabrainz.org/t/467885/5

The disc ID is removed if an “Add Medium” edit is cancelled after using “CD stub import”, but there is a bug here, too: Even though the cancellation removes the disc ID from the release, it does not return the disc ID to the database as an available stub. The stub is gone forever. If that happens, one can only recover the TOC metadata by copying it from the cancelled edit and adding it to a release manually.