Issue upgrading from 9.5 to 13 with pg_upgrade: "connection to database failed: FATAL: database "template1" does not exist"

I’m running CentOS 8 on an EC2 instance and attempting to upgrade a 9.5 database to 13 using pg_upgrade. Both are running on the same box and pass initial tests but it fails during the later part of the process.

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bash-4.4$ /usr/pgsql-13/bin/pg_upgrade --old-bindir /usr/pgsql-9.5/bin --new-bindir /usr/pgsql-13/bin --old-datadir /var/lib/pgsql/9.5/data --new-datadir /var/lib/pgsql/13/data --old-port=54320 --new-port=5432 --socketdir=/var/run/postgresql/
Performing Consistency Checks
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Checking cluster versions ok
Checking database user is the install user ok
Checking database connection settings ok
Checking for prepared transactions ok
Checking for reg* data types in user tables ok
Checking for contrib/isn with bigint-passing mismatch ok
Checking for tables WITH OIDS ok
Checking for invalid "sql_identifier" user columns ok
Checking for invalid "unknown" user columns ok
Checking for roles starting with "pg_" ok
Creating dump of global objects ok
Creating dump of database schemas
ok

connection to database failed: FATAL: database "template1" does not exist

could not connect to target postmaster started with the command:
"/usr/pgsql-13/bin/pg_ctl" -w -l "pg_upgrade_server.log" -D "/var/lib/pgsql/13/data" -o "-p 5432 -b -c synchronous_commit=off -c fsync=off -c full_page_writes=off -c vacuum_defer_cleanup_age=0 -c listen_addresses='' -c unix_socket_permissions=0700 -c unix_socket_directories='/var/run/postgresql'" start
Failure, exiting

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When I manually run that command it works, although when I attempt to connect to the ‘postgres’ database on it, it complains it doesn’t exist.

I can’t use the dump/restore method to upgrade this server as the data in question is fairly massive and in my testing it took 45 hours to complete. Any help appreciated.