StepFunctions not being triggered when adding object to existing S3 bucket

Serverless Framework
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I’m trying to set up a Serverless application in which objects added to an existing S3 bucket would trigger jobs in an existing StepFunctions state machine via a Lambda function. This works successfully when I deploy the application without using an existing S3 bucket, but rather using the target bucket the application creates. However, when I point to an existing S3 bucket, the stack gets created successfully, but no jobs are triggered in the state machine.

I tried getting this to work by 1) using the serverless-plugin-existing-s3 plugin and by 2) setting existing: true under the - s3 event definition in the yml file, but neither work. Here are my serverless.yml files I used for these 2 cases:

    service: cfn-trigger-test
    frameworkVersion: '2'

    provider:
      name: aws
      runtime: python3.8
      stage: dev
      region: us-west-1

    functions:
      cfnPipelineTrigger:
        handler: handler.upload
        events:
          - existingS3:
              bucket: cfnbuckets-samples
              event: s3:ObjectCreated:*
        environment:
          MY_STATE_MACHINE_ARN: arn:aws:states:us-west-1:{AccountNum}:stateMachine:cfnpipeline
        iamRoleStatements:
          - Effect: "Allow"
            Action:
              - states:*
            Resource: "*"


    plugins:
      - serverless-step-functions
      - serverless-iam-roles-per-function
      - serverless-plugin-existing-s3

    package:
      individually: true
      exclude:
        - '**/*'
      include:
        - handler.py

    service: cfn-trigger-test
    frameworkVersion: '2'

    provider:
      name: aws
      runtime: python3.8
      stage: dev
      region: us-west-1

    functions:
      cfnPipelineTrigger:
        handler: handler.upload
        events:
          - s3:
              bucket: cfnbuckets-samples
              event: s3:ObjectCreated:*
              existing: true
        environment:
          MY_STATE_MACHINE_ARN: arn:aws:states:us-west-1:{AccountNum}:stateMachine:cfnpipeline
        iamRoleStatements:
          - Effect: "Allow"
            Action:
              - states:*
            Resource: "*"


    plugins:
      - serverless-step-functions
      - serverless-iam-roles-per-function

    package:
      individually: true
      exclude:
        - '**/*'
      include:
        - handler.py