I think this might be the same issue as of https://github.com/elastic/elasticsearch/pull/22174
I tested it with 5.3 and master and it looks like it's fixed.
As a workaround you can add the explicit date format to the dynamic mapping for the date field:
"date_fields": {
"mapping": {
"type": "date",
"format": "yyyy/MM/dd",
"doc_values": true
},
"match": "*",
"match_mapping_type": "date"
}