Interesting. At a previous company, when we changed the pg_dump command in a backup script to start using parallel workers (-j flag) we started to rarely see errors that suggested inconsistency when restoring the backups (duplicate key errors and fk constraint errors). At the time, I tried reporting the issue to both AWS and on the Postgres mailing list but never got anywhere since I could not easily reproduce it. We eventually gave up and went back to single threaded dumps. I wonder if this issue is related to that behavior we were seeing.
We saw it when we ran the pg_dump off a standby instance (or a "replica" to use RDS terminology). Our primary was a multi-az instance. So not exactly what they tested here I guess, but it makes me wonder what changes, if any, they've made to postgres under the hood.
