Fund: Malformatted custom scalar default arg raises error even if valid arg provided in `variables`

Describe the Bug

A GraphQL query like the following (which GraphiQL's explorer will generate by default if you chose to use variables):

mutation MyMutation($input: MyMutationInput! = {date: ""}) {
  echoDate(input: $input) {
    myDate
  }
}

Will receive a response of:

{
  "data": null,
  "errors": [
    {
      "message": "Value cannot represent a Date: \"\". Invalid isoformat string: ''"
    }
  ]
}

Even when a variables argument is provided with a correctly formatted argument:

{
  "input": {
    "date": "2023-05-31"
  }
}

I'm not sure if this should be considered a bug or proper functionality. If the latter, I think at least a note of caution in the docs is called for.