To identify the root cause of the upgrade failure we would need to see the Kibana logs at the time when Kibana was upgraded (sharing the logs after restarting Kibana won't be useful as once a migration has failed, Kibana won't retry the migration until the migration lock has been removed).
For instructions to remove a migration lock to allow Kibana to retry the migration see https://www.elastic.co/guide/en/kibana/current/upgrade-migrations.html