AWS Amplify API authentication error for unauthenticated user

Question

I created a books API with Amplify for an iOS app. I want the book table to have read access for both authenticated and unauthenticated users. Here's the schema:

type Book
  @model
  @searchable
  @auth(
    rules: [
      {allow: private, provider: userPools, operations: [read]},
      {allow: groups, groups: ["Admin"]}
    ]
  )
{
  id: ID!
  category: BookCategoryType!
  description: String
  groupsCanAccess: [String]!
  images: [Image!]
    @auth(rules: [
        {allow: groups, groupsField: "groupsCanAccess", operations: [read]},
        {allow: groups, groups: ["Admin"]}
    ])
  title: String!
}

However, when calling fetch from AWSAppSyncClient, I get the following error:

authenticationError(AWSMobileClient.AWSMobileClientError.notSignedIn(message: "User is not signed in, please sign in to use this API."))

The Cognito identity pool has Enable access to unauthenticated identities checked and there's an unauth IAM role.

What could be the problem?