Amazon RDS for PostgreSQL is an Amazon Web Services (AWS) service which provides managed instances of the PostgreSQL database. We show that Amazon RDS for PostgreSQL multi-AZ clusters violate Snapshot Isolation, the strongest consistency model supported across all endpoints. Healthy clusters occasionally allow Long Fork and other G-nonadjacent cycles. These phenomena occurred in every version tested, from 13.15 to 17.4. Amazon RDS for PostgreSQL may instead provide Parallel Snapshot Isolation. T...