Creating an RDS database in a custom availability zone fails with invalid availability zone

[rmauge]

Describe the bug
We have a custom availability zone that is hosted on-prem on VMware and are trying to use the service broker tile on Tanzu (Pivotal) Application Service. The command to create the service is able to submit the request to start the creation of the instance. But creating the service fails with the message found in the CloudFormation logs:

Value (custom-az-name)  for parameter availabilityZone is invalid. Subnets can currently only be created in the following availability zones: us-east-1a, us-east-1b, us-east-1c, us-east-1d, us-east-1e, us-east-1f. (Service: AmazonEC2; Status Code: 400; Error Code: InvalidParameterValue; Request ID: ...)

To Reproduce
The AWS Broker Tile is correctly configured in OpsMan and its services are available. Create a new service instance using:

cf create-service rdsmysql custom cf-mysql-onprem -c rds.json

rds.json

{
 "AccessCidr": "0.0.0.0/0",
 "MasterUsername": "admin",
 "VpcId": "vpc...",
 "region": "us-east-1",
 "AvailabilityZones": "custom-az-name",
 "NumberOfAvailabilityZones": "1"
}

Expected behavior
The database instance should be successfully created in RDS

Environment (please complete the following information):

• Application Platform: Tanzu Application Service
• Application Platform Version: v2.8.2-build.203
• Broker Version: AWS Service Broker v1.0.2

Additional context
We can successfully create RDS instances when the az is from the native az set using the same commands but the creation of these only work when at least 2 azs are supplied to the configuration.

We can also directly create dbs using the RDS console.