View Surrounding Documents causes failed to parse date field exception

[w_batelaan]

Using ES6.4.0; Discover works fine with this index, which has a @timestamp field, which Kibana shows as
September 4th 2018, 15:41:08.309

So why would View Surrounding Documents not be able to parse the date?
BTW the mapping for @timestamp is this:
"@timestamp": {
"type": "date",
"format": "dateOptionalTime"
}

Unable to load documents

Request to Elasticsearch failed: {"error":{"root_cause":[{"type":"parse_exception","reason":"failed to parse date field 1535985668309 with format dateOptionalTime"}],"type":"search_phase_execution_exception","reason":"all shards failed","phase":"can_match","grouped":true,"failed_shards":[{"shard":0,"index":"logstash6-2018.01","node":"EDi1wHveRX-MVAQVKm1K4g","reason":{"type":"parse_exception","reason":"failed to parse date field 1535985668309 with format dateOptionalTime","caused_by":{"type":"illegal_argument_exception","reason":"Unrecognized chars at the end of 1535985668309: 8309"}}}]},"status":400}

[Bargs (Matt Bargar)]

Hi @w_batelaan, unfortunately this is a bug in 6.4. I just submitted a pull request to fix the issue: https://github.com/elastic/kibana/pull/22684

If you need to work around it in the meantime, you could update your mapping for that field to use the format date_optional_time||epoch_millis.