AWS SAM cors error for 5XX timeout thought mentioned in Template

refer bellow template

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >-
  aws-sam

Globals:
  Function:
    Runtime: nodejs16.x
    CodeUri: .
    Architectures:
      - arm64
    Timeout: 120
    MemorySize: 1024

Resources:
  GatewayResponseDefault5XX:
    Type: 'AWS::ApiGateway::GatewayResponse'
    Properties:
      ResponseParameters:
         gatewayresponse.header.Access-Control-Allow-Origin: "'*'"
         gatewayresponse.header.Access-Control-Allow-Headers: "'*'"
      ResponseType: DEFAULT_5XX
      RestApiId:
        Ref: 'ApiGatewayApi'
  ApiGatewayApi:
    Type: AWS::Serverless::Api
    Properties:
      StageName: staging
      Cors:
        AllowOrigin: "'*'"
        AllowMethods: "'POST, OPTIONS'"
        AllowHeaders: "'*'"
  lambdaFunction:
    Type: 'AWS::Serverless::Function'
    Properties:
      FunctionName: lambdaFn
      Handler: src/handlers/lambdaFn.handler
      Events:
        PublicFunctionCreate:
          Type: Api
          Properties:
            Path: /public/create
            Method: post
            RestApiId:
              Ref: ApiGatewayApi

the above SAM template is used and works perfectly and from lambda function I return

  return {
    statusCode: 200,
    headers: {
      'Access-Control-Allow-Origin': '*',
      'Access-Control-Allow-Methods': 'OPTIONS,POST',
      'Access-Control-Allow-Headers': '*',
    }
  }

which works in normal case but since my lambda function runs for more than 30sec, the API gateway timeout after 30sec which is expected and returns 504 error since the 'Access-Control-Allow-Origin' header is missing in the response header due to which the browser gives cors error.

Though in template I have mentioned DEFAULT_5XX response header it doesn't work.

Am I missing something or it is an issue with AWS SAM