Search_after with sort on nested fields

OpenSearch
,
1

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser):
Opensearch 2.9

Describe the issue:
I’ve encountered an issue with search_after functionality used with sort on nested field.

I have around 40k documents and I want to retrieve them using search_after

I expect that I will eventually have 30k records at the end (after all search_after queries), example search_after query:

{
  "query: { some query here },
  "search_after": [
    123,
    "some_id"
  ],
  "sort": [
    {
      "AAA.bbb": {
        "order": "asc",
        "unmapped_type": "long",
        "nested": {
          "path": "AAA"
        }
      }
    },
    {
      "_id": {
        "order": "asc"
      }
    }
  ]
}

as a result I have ~11k results.

but for the same query but different sort params (not on nested field) I have proper expected results (30k), example query:

      "search_after": [
        "some_name",
        "some_id"
      ],
      "sort": [
        {
          "name": {
            "order": "desc",
            "unmapped_type": "long"
          }
        },
        {
          "_id": {
            "order": "asc"
          }
        }
      ]

Has anyone had a similar issue or have an idea what could be the problem?

Thanks in advance !
Pawel