AWS RDS (PostgreSQL) - Master user lost all privileges after upgrading from v9.6 to v10.6-R1
So running these two statements resolved my issue (being unable to access anything in the public schema):

master_prod=&gt; ALTER SCHEMA public OWNER to freetiersuperuser;
ALTER SCHEMA
master_prod=&gt; GRANT ALL ON SCHEMA public TO freetiersuperuser;
GRANT


However my rds_superuser role's attributes haven't changed, and I would definitely like to understand if this is normal (I don't think it is):

     Role name     |                         Attributes                         |                          Member of
-------------------+------------------------------------------------------------+-------------------------------------------------------------
 freetiersuperuser | Create role, Create DB                                    +| {rds_superuser}
                   | Password valid until infinity                              |
 rds_iam           | Cannot login                                               | {}
 rds_password      | Cannot login                                               | {}
 rds_replication   | Cannot login                                               | {}
 rds_superuser     | Cannot login                                               | {pg_monitor,pg_signal_backend,rds_replication,rds_password}
 rdsadmin          | Superuser, Create role, Create DB, Replication, Bypass RLS+| {}
                   | Password valid until infinity                              |
 rdsrepladmin      | No inheritance, Cannot login, Replication                  | {}