Alias values are getting null in appsync subscriptions

What is the reason for getting null in subscription response, getting proper value in mutation response

subscription MySubscription {
  updateFieldResponseSubscription(id: 1){
    __typename
    ... on TaskFieldResponse {
      id
      fieldId
      field {
        response {
          ... on GQLStringType {
            stringVal: value
            __typename
          }
          __typename
        }
        __typename
      }
      __typename
    }
  }
}

Here I am getting null for the stringVal

My Mutation is

mutation MyMutation($inputArguments: UpdateFieldResponseArgs!) {
  updateFieldResponse(inputArguments: $inputArguments){
    __typename
    ... on TaskFieldResponse {
      id
      fieldId
      field {
        response {
          ... on GQLStringType {
            stringVal: value
            __typename
          }
          __typename
        }
        __typename
      }
      __typename
    }
  }
}