Source filtering fails for nested fields under backslash-named parent field

Elasticsearch Version

9.1.5

Installed Plugins

No response

Java Version

bundled

OS Version

Darwin Kernel Version 24.6.0: Mon Aug 11 21:16:34 PDT 2025; root:xnu-11417.140.69.701.11~1/RELEASE_ARM64_T6020 arm64

Problem Description

Source filtering using _source parameter fails to return nested fields when the parent field name is a backslash (\) character. This worked correctly in version 8.5.3 but stopped working starting from 8.6.0.

Steps to Reproduce

1. Create an index with a backslash-named field containing nested properties:

PUT test
{
  "mappings": {
    "properties": {
      "\\": {
        "properties": {
          "nested_value": {
            "type": "text"
          }
        }
      }
    }
  }
}

2. Index a document:

POST test/_doc?refresh
{
  "\\": {
    "nested_value": "value_C"
  }
}

3. Query with source filtering for the parent field (works correctly):

GET test/_search
{
  "_source": ["\\"]
}

Result: Returns the full \ object with its nested_value ✅

4. Query with source filtering for the nested field (fails):

GET test/_search
{
  "_source": ["\\.nested_value"]
}

Result: Returns empty source ❌

Expected behavior: The query in step 4 should return:

{
  "hits": {
    "hits": [
      {
        "_source": {
          "\\": {
            "nested_value": "value_C"
          }
        }
      }
    ]
  }
}

Actual behavior: The query returns documents but with empty _source:

{
  "hits": {
    "hits": [
      {
        "_source": {}
      }
    ]
  }
}

• This is a regression: the same query worked correctly in version 8.5.3. It still does not work in 9.1.5
• The issue specifically affects field paths where the parent field name is a backslash character
• Fetching the parent field \ directly works, but accessing nested properties via dot notation fails

Logs (if relevant)

No response