[BUG] `head` on struct removes other fields in that struct

[Swiddis]

What is the bug?
For a struct field "a": { "b": 1, "c": 2 }, a query with fields a, a.b, a.c | head only returns an a column.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Index a single big5 doc (here's one with irrelevant fields removed):

{
   "@timestamp":"2023-01-01T00:57:41.000Z",
   "agent":{
      "id":"baac7358-a449-4c36-bf0f-befb211f1d38",
      "name":"crystaldrifter",
      "type":"filebeat",
      "version":"8.8.0",
      "ephemeral_id":"baac7358-a449-4c36-bf0f-befb211f1d38"
   }
}

2. Try a query that mixes struct sub-fields and the struct field. This correctly returns all 3 referenced fields:

// source=big5 | fields agent.id agent.name agent
{
  "schema": [
    {
      "name": "agent.id",
      "type": "string"
    },
    {
      "name": "agent.name",
      "type": "string"
    },
    {
      "name": "agent",
      "type": "struct"
    }
  ],
  "datarows": [
    [
      "baac7358-a449-4c36-bf0f-befb211f1d38",
      "crystaldrifter",
      {
        "name": "crystaldrifter",
        "id": "baac7358-a449-4c36-bf0f-befb211f1d38",
        "type": "filebeat",
        "ephemeral_id": "baac7358-a449-4c36-bf0f-befb211f1d38",
        "version": "8.8.0"
      }
    ]
  ],
  "total": 1,
  "size": 1
}

3. Now add a head: The first two fields disappear.

// source=big5 | fields agent.id agent.name agent | head
{
  "schema": [
    {
      "name": "agent",
      "type": "struct"
    }
  ],
  "datarows": [
    [
      {
        "name": "crystaldrifter",
        "id": "baac7358-a449-4c36-bf0f-befb211f1d38",
        "type": "filebeat",
        "ephemeral_id": "baac7358-a449-4c36-bf0f-befb211f1d38",
        "version": "8.8.0"
      }
    ]
  ],
  "total": 1,
  "size": 1
}

What is the expected behavior?
fields should return all columns listed. Extracting redundant sub-fields from a struct is sometimes useful for presentation ("I just want to scan this one key before digging into the whole struct")

What is your host/environment?

• mainline (JDK 25)

Do you have any screenshots?
N/A

Do you have any additional context?
N/A