How to properly transform backend error responses using response-transformer or custom policy in Express Gateway?

I'm working with Express Gateway to proxy requests to a backend service and want to transform error responses into a common format like this:

const errorResponse = {
  timestamp: new Date().toISOString(),
  status: status,
  errorCode: errorCode,
  message: message,
  details: details
};

I tried using the built-in response-transformer policy in my gateway.config.yaml:

pipelines:
  default:
    apiEndpoints:
      - companyApi
    policies:
      - log:
          - action:
              message: 'Request received: ${req.method} ${req.url}'
      - proxy:
          - action:
              serviceEndpoint: backendService
              changeOrigin: true
      - response-transformer:
          - action:
              body:
                # Tried adding transformations here but no effect

But the backend response is returned as-is; no transformation happens. Then I also tried creating a custom response-wrapper policy to intercept and change the response, but it seems that middleware is not applied or changing the response either.

response-wrapper policy

policy: () => {
    return (req, res, next) => {
      const originalSend = res.send.bind(res);
      res.send = (body) => {
        let backendMessage;
        try {
          backendMessage = typeof body === 'string' ? JSON.parse(body) : body;
        } catch (e) {
          backendMessage = body.toString();
        }
        const wrappedResponse = {
          status: 'success',
          message: backendMessage
        };
        res.set('Content-Type', 'application/json');
        return originalSend(JSON.stringify(wrappedResponse));
      };
      next();
    };
  }

How can I properly intercept and transform error responses coming from a proxied backend in Express Gateway? Is response-transformer not suitable for this scenario or am I missing something in configuration?