Expected behavior that overlapping interfaces leak into each other's fields?

[jeffsee55]

If I have a type Post and it has a field author which is an interface (PostAuthor), a type which implements the PostAuthor interface leaks all of the other interfaces it implements into the post.author field. I was surprised this is considered valid by GraphQL, is this expected behavior?

interface Node {
  id: String!
}

interface Document implements Node {
  id: String!
  filename: String!
}

type Post implements Node & Document {
  id: String!
  filename: String!
  title: String!
  author: PostAuthor!
}

type Page implements Node & Document {
  id: String!
  filename: String!
  title: String!
  author: PageAuthor!
}

interface PostAuthor {
  postDetails: String!
}

interface PageAuthor {
  pageDetails: String!
}

type Author implements Node & Document & PostAuthor & PageAuthor {
  id: String!
  filename: String!
  name: String!
  postDetails: String!
  pageDetails: String!
}

type Editor implements Node & Document & PostAuthor {
  id: String!
  filename: String!
  editorName: String!
  postDetails: String!
}

type Query {
  document: Document
  post: Post
}

type Mutation {
  updateDocument(args: String!): Document
}

schema {
  query: Query
  mutation: Mutation
}

This is a valid query:

{
  post {
    author {
      ...on PostAuthor {
        postDetails
      }
      # this only exists because Author implements it
      ...on PageAuthor {
        pageDetails
      }
    }
  }
}

If you comment out the type Author in the schema, this query is no longer valid