Search after doesn't work with date type. Is it bug?

[Vitaly_Igorevich]

Elasticsearch 6.1
Mapping:
{
"posts" : {
"mappings" : {
"post" : {
"dynamic" : "false",
"properties" : {
"created_at" : {
"type" : "date",
},
"format" : "epoch_second"
},
}
}
}
}
}

Query:
{
"size":25,
"query":{
"bool":{
"filter":[

         ]
      }
   },
   "sort":[
      {
         "created_at":"desc"
      },
      {
         "_id":"desc"
      }
   ],
   "search_after":[
      1513608331,
      "2"
   ]
}

This query return empty result. But when i create field with type keyword like this it works well. Is it bug?
Mapping:
{
"posts" : {
"mappings" : {
"post" : {
"dynamic" : "false",
"properties" : {
"created_at" : {
"type" : "date",
"fields" : {
"keyword" : {
"type" : "keyword"
}
},
"format" : "epoch_second"
},
}
}
}
}
}

Query:
{
"size":25,
"query":{
"bool":{
"filter":[

         ]
      }
   },
   "sort":[
      {
         "created_at.keyword":"desc"
      },
      {
         "_id":"desc"
      }
   ],
   "search_after":[
      1513608331,
      "2"
   ]
}