Scan results are not being filtered correctly in response mapping

I have a simple test table with user data with the following schema

type GlobalUsers {
	userid: String!
	firstname: String
	lastname: String
	email: AWSEmail
	phone: AWSPhone
	public: Boolean
}

The listGlobalUsers query with an unfiltered response mapping returns

{
  "data": {
    "listGlobalUsers": {
      "items": [
        {
          "userid": "janedoe",
          "email": "jane.doe@gmail.com",
          "phone": "12345678901"
        },
        {
          "userid": "johndoe",
          "email": "john.doe@gmail.com",
          "phone": "12345678901"
        }
      ]
    }
  }
}

If I modify the response template to filter on items marked as public

#set($publicRecords = [])
#foreach($item in $ctx.result.items)
    #if($item.public == true)
        #set($added = $publicRecords.add($item))
    #end
#end
$utils.toJson($publicRecords)

The query results are always null

{
  "data": {
    "listGlobalUsers": {
      "items": null
    }
  }
}

I wonder if I have something configured incorrectly because the results are always null, even if I remove the condition and add all items to $publicRecords