[BUG] Search request with PIT + searchAfter and 2 sort fields loses some hits

[Anastasiia186]

Describe the bug

#15869

Executing a search request with a PIT + searchAfter and 2 sort fields (first one is not uniq, second one is uniq) returns less hits in total that for track_total_hits. The smaller the page size, the more data is lost, if you use scrolling, there is no problem. if you change the sorting values (only 1 uniq or first one is uniq and second one is not uniq), the problem also disappears

Related component

Clients

To Reproduce

How can one reproduce the bug?

create a search request with a PIT and 2 sort fields like

 PointInTimeBuilder pointInTimeBuilder = new PointInTimeBuilder(pitId);
 SearchSourceBuilder searchSourceBuilder = new SearchSourceBuilder()
                .pointInTimeBuilder(pointInTimeBuilder);
        searchSourceBuilder.query(<query>);
        searchSourceBuilder.sort(SortBuilders
                .fieldSort(<not uniq field>)
                .order(SortOrder.ASC));
        searchSourceBuilder.sort(SortBuilders
                .fieldSort(<uniq fiel>)
                .order(SortOrder.ASC));
        searchSourceBuilder.size(10_000)

_____________

"sort": [
  {
      "<not uniq field>":{
        "order": "asc"
      }
    },
    {
      "<uniq fiels>": {
        "order": "asc"
      }
    }
  ]

run this query several times using searchAfter like:

searchSourceBuilder.searchAfter(prevSearch.getHits().getHits()[prevSearch.getHits().getHits().length - 1].getSortValues());
SearchRequest searchRequest = new SearchRequest();
searchRequest.source(searchSourceBuilder);
restHighLevelClient.search(searchRequest, RequestOptions.DEFAULT)

and notice that in total you have less hits that returns in track_total_hits:
total using PIT = 1999953
"total": {
"value": 2000000,
"relation": "eq"
}

Expected behavior

A search response in total returns the same number of hits as for track_total_hits

Additional Details

SpringBoot application

[dblock]

@Anastasiia186 Want to try and turn this into a failing YAML REST test?

[Anastasiia186]

@dblock we rechecked the second field and found some duplicates...we removed them and now it works fine
thank you!