GraphQL Schema Rules Work Individually but Not When Combined

Hello,

Requirement
Users can only borrow a book that is AVAILABLE and can only borrow for themselves.

Rules
I’ve created the following rules:

{
  "%%true": {
    "%function": {
      "name": "isBookAvailable",
      "arguments": [
        "%%root.book"
      ]
    }
  },
  "user": {
      "%stringToOid": "%%user.id"
  }
}

As mentioned in the title, when I remove either of the rules it will work. However when combined it will always fail. My implementation is based on Multi-Field Expressions.

Cheers!