“We show that Amazon RDS for PostgreSQL multi-AZ clusters violate Snapshot Isolation, the strongest consistency model supported across all endpoints.”
Jepsen found Amazon RDS multi-AZ PostgreSQL clusters violate snapshot isolation on healthy clusters under moderate load. Every version from 13 to 17 did it. The multi-AZ cluster is less safe than a single PostgreSQL node. The cause was described on a mailing list in 2013.