Obtain GitHub issues in a specific milestone using GraphQL and IssueFilters

Question

A similar question was previously asked but doesn't use IssueFilters which is what I want to use in this question.

Using GitHub's GraphQL Explorer, I am able to get issues of a repository using this query:

{
  repository(owner: "neovim", name: "neovim") {
    hasIssuesEnabled
    issues(first: 20, orderBy: {field: CREATED_AT, direction: DESC}, filterBy: {milestone:"*"}) {
      nodes {
        ... on Issue {
          number
          title
          milestone {
            number
      id
      url
      title
          }
        }
      }
    }
  }
}

The milestone is an issueFilter which, according to the documentation, allows you to:

List issues by given milestone argument. If a string representation of an integer is passed, it should refer to a milestone by its number field. Pass in null for issues with no milestone, and * for issues that are assigned to any milestone.

However, using a filter that is not a * but let's say filterBy: {milestone:"9"} (9 being a valid milestone number for the repository I am using), the query returns no nodes:

{
  "data": {
    "repository": {
      "hasIssuesEnabled": true,
      "issues": {
        "nodes": []
      }
    }
  }
}

I do not understand why this does not work from the documentation I read. Am I missing something regarding what means "it should refer to a milestone by its number field"?