Elasticsearch nested sort problem

commands.json

PUT /testindex
{
   "settings": {
      "number_of_replicas": 0,
      "number_of_shards": 1
   },
   "mappings": {
      "testtype": {
         "dynamic": "strict",
         "properties": {
            "organisation": {
               "type": "object",
               "properties": {
                  "code": {
                     "type": "string",
                     "index": "not_analyzed"
                  }
               }
            },
            "publications": {
               "type": "nested",
               "properties": {
                  "dateEnd": {
                     "type": "long"
                  },
                  "dateStart": {
                     "type": "long"
                  },
                  "code": {
                     "type": "string",
                     "index": "not_analyzed"
                  }
               }
            }
         }
      }
   }
}

PUT /testindex/testtype/a
{
   "organisation": {
      "code": "01310"
   },
   "publications": [
      {
         "dateEnd": 5,
         "dateStart": 1,
         "code": "PUB.02"
      },
      {
         "dateEnd": 5,
         "dateStart": 2,
         "code": "PUB.06"
      }
   ]
}

PUT /testindex/testtype/b
{
   "organisation": {
      "code": "01312"
   },
   "publications": [
      {
         "dateEnd": 5,
         "dateStart": 1,
         "code": "PUB.02"
      },
      {
         "dateEnd": 5,
         "dateStart": 2,
         "code": "PUB.06"
      }
   ]
}

GET /testindex/testtype/_search
{
   "sort": {
      "publications.dateStart": {
         "mode": "min",
         "order": "desc",
         "nested_filter": {
            "or": [
               {
                  "and": [
                     {
                        "term": {
                           "organisation.code": "01310"
                        }
                     },
                     {
                        "term": {
                           "publications.code": "PUB.02"
                        }
                     }
                  ]
               },
               {
                  "term": {
                     "publications.code": "PUB.06"
                  }
               }
            ]
         }
      }
   }
}