Issues when using @interfaceObject in combination with fragments

[oskargotte]

Issue Description

Consider these simplified schemas from the interface extension example where Subgraph B extends the Media interface and hence the implementing Book type from Subgraph A with an extra review field.

Subgraph A	Subgraph B
interface Media @key(fields: "id") { id: ID! title: String! } type Book implements Media @key(fields: "id"){ id: ID! title: String! }	type Media @key(fields: "id") @interfaceObject { id: ID! review: Int! } type Query { topRatedMedia: [Media!]! }

One can query for Media with title from Subgraph A and review from Subgraph B like this

topRatedMedia {
  title
  review
}

One can use a fragment to get the review field only for Book types in case there are more implementations of the Media interface

topRatedMedia {
  title
  ... on Book {
    review
  }
}

However, if __typename fields are added to the previous query (which is what the Apollo Client would do)

topRatedMedia {
  __typename
  title
  ... on Book {
    __typename
    review
  }
}

the query fails silently with the data being null, no error in the response and no error in the router logs but an extension in the response

{"data":null,"extensions":{"valueCompletion":[{"message":"Cannot return null for non-nullable array element of type Media at index 0","path":["topRatedMedia",0]},{"message":"Cannot return null for non-nullable field [Media!]!.topRatedMedia","path":["topRatedMedia"]}]}}

Link to Reproduction

https://codesandbox.io/p/devbox/nervous-silence-5rpvys

Steps to reproduce

Open the sandbox link above and try the different queries

[oskargotte]

Closely related to #2743 which is solved for the queries mentioned in the issue but would still fail on e.g.

  searchProducts {
    id
    ... on Book {
      __typename
      reviews
    }  
  }

Seems like an issue with the query plan.