Following migrating Amplify CLI and AppSync Transformer to v2 no longer able to access CloudFormation parameters from a custom resource

Question

We have an AWS Amplify project that I am in the process of migrating the API from Transformer 1 to 2.

As part of this, we have a number of custom resolvers that previously had their own stack JSON template in the stacks/ folder as generated by the Amplify CLI.

As per the migration instructions, I have created new custom resources using amplify add custom which allow me to create either CDK (Cloud development kit) resource or a CloudFormation template. I just want a lift n shift for now so I've gone with the template option and moved the content from the stack JSON to the new custom resolver JSON template.

This seems like it should work, but the custom templates no longer have access to the parameters shared from the parent stack:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Metadata": {},
  "Parameters": {
    "AppSyncApiId": {
      "Type": "String",
      "Description": "The id of the AppSync API associated with this project."
    },
    "S3DeploymentBucket": {
      "Type": "String",
      "Description": "The S3 bucket containing all deployment assets for the project."
    },
    "S3DeploymentRootKey": {
      "Type": "String",
      "Description": "An S3 key relative to the S3DeploymentBucket that points to the root of the deployment directory."
    }
  },
  ...
}

So these are standard parameters that were used previously and my challenge now is in accessing the deployment bucket and root key as these values are generated upon deployment.

The exact use case is for the AppSync function configuration when I attempt to locate the request and response mapping template S3 locations:

"RequestMappingTemplateS3Location": {
  "Fn::Sub": [
"s3://${S3DeploymentBucket}/${S3DeploymentRootKey}/resolvers/MyCustomResolver.req.vtl",
    {
      "S3DeploymentBucket": {
        "Ref": "S3DeploymentBucket"
      },
      "S3DeploymentRootKey": {
        "Ref": "S3DeploymentRootKey"
      }
    }
  ]
 },

The error message I am receiving is AWS::CloudFormation::Stack Tue Feb 15 2022 18:20:42 GMT+0000 (Greenwich Mean Time) Parameters: [S3DeploymentBucket, AppSyncApiId, S3DeploymentRootKey] must have values

I feel like I am missing a step to plumb the output values to the parameters in the JSON but I can't find any documentation to suggest how to do this using the updated Amplify CLI options.

Let me know if you need any further information and fingers crossed it is something simple for you Amplify/CloudFormation ninjas out there!

Thank you in advance!