Interesting. Can you nail down the software versions that were in use here? That'd be the old PG server version upgraded from, the new server version upgraded to, the versions of pg_upgrade and pg_dump (these probably should match the new server version, but I'm not certain we enforce that), and the pg_upgradecluster script version?
Well, it was an upgrade from 9.5 to 10. Unfortunately I can't pin down any other or more exact versions any longer. The command we used was "pg_upgradecluster 9.5 main -m upgrade -k -v 10"
