[BUG] No hits for keys following an array in a flat_object

[marko-metaps]

Describe the bug

When using the flat_object type, keys that appear after an array within the object do not return any hits in search queries

Related component

Other

To Reproduce

1. create an index:

curl -X PUT "$OS_HOST/$INDEX_NAME" -H 'Content-Type: application/json' -d '{
    "mappings": {
      "properties": {
        "data": {
          "type": "object",
          "properties": {
            "detail": {
              "type": "flat_object"
            }
          }
        }
      }
    }
  }'

2. Add data

curl -X POST "$OS_HOST/$INDEX_NAME/_doc/1" -H 'Content-Type: application/json' -d '{
    "data": {
      "detail": {
        "val1": "val1",
        "array1": [
          {
            "key3": "value5",
            "key4": "value6"
          },
          {
            "key3": "value7",
            "key4": "value8"
          }
        ],
        "val2": "val2"
      }
    }
  }'

3. Query the data:

curl -X GET "$OS_HOST/$INDEX_NAME/_search" -H 'Content-Type: application/json' -d '{
    "query": {
      "match": {
        "data.detail.val2": "val2"
      }
    }
  }'

4. It retrieves 0 hits, although searching the value before the array works fine.

curl -X GET "$OS_HOST/$INDEX_NAME/_search" -H 'Content-Type: application/json' -d '{
    "query": {
      "match": {
        "data.detail.val1": "val1"
      }
    }
  }'

Expected behavior

Query should return matching values, despite the structure of the flat_object.

Additional Details

Plugins
No plugins enabled

Screenshots

Host/Environment (please complete the following information):

• OS: Mac OS Sonoma 14.1.1
• Version 3.0.0-SNAPSHOT

Additional context
Add any other context about the problem here.

[dblock]

Looks like a legit problem. Want to turn this into a (failing) YAML REST test?