GraphQL requests erroring with "must contain a non-empty `query`"

When attempting to send a GraphQL query, I am getting the following error:

{
  "errors": [
    {
      "message": "GraphQL operations must contain a non-empty `query` or a `persistedQuery` extension.",
      "extensions": {
        "http": {
          "status": 400,
          "headers": {}
        },
        "code": "BAD_REQUEST"
      }
    }
  ]
}

If I change the request type to JSON, it then works. The issue seems to be intermittent - sometimes changing back to GraphQL then works. Sometimes not.