There's a rare situation (but one which we have hit a few times, as per Sentry, https://metabrainz-foundation-inc.sentry.io/issues/44660512/) where a track gets associated with a deleted recording (either the search index returns a deleted recording still in the index, or possibly the recording gets removed after the release editor has been loaded). This then causes an ISE when process_medium tries to process the tracks, since get_by_gids returns no recording and as such $recordings->{$recording_gid} is undefined, but we try to call ->id from it.