RDS Proxy works with psycopg2 but not SQLAlchemy [duplicate]

I'm connecting to an RDS Instance running PostgreSQL through a proxy.

I'm able to connect the database without issue when I run it through psycopg2 using the code below

creds = read_config(config_file_location)
rds_client = boto3.client('rds')

auth_token = rds_client.generate_db_auth_token( 
    DBHostname=creds['host'], 
    Port=creds['port'], 
    DBUsername=creds['user'],
    Region=creds['region']
)

return psycopg2.connect(user=creds['user'],
                        password=auth_token,
                        host=creds['host'],
                        port=creds['port'],
                        database=creds['db'],
                        sslmode='require')

But when I try with SQLAlchemy

config = read_config(config_file_location)
rds_client = boto3.client('rds')

auth_token = rds_client.generate_db_auth_token( 
    DBHostname=config['host'], 
    Port=config['port'], 
    DBUsername=config['user'],
    Region=config['region']
)

config['password'] = auth_token
sqlalchemy_url = "postgresql://{user}:{password}@{host}:{port}/{db}".format(**config)
engine = create_engine(sqlalchemy_url(), connect_args={"sslmode": "require"})

This throws an error

sqlalchemy.exc.OperationalError: (psycopg2.OperationalError) connection to server 
at "<<PROXY HOST>>" (<< IP >>), port 5432 failed: 
FATAL:  The IAM authentication failed for the role << USER >>. 
Check the IAM token for this role and try again.

The SQLAlchemy connection works if I connect directly to the DB using the password

Not sure only Proxy + Alchemy fails. Any suggestions are welcome