Return date fields as long numbers when using synthetic _source and having format of date as epoch_millis

Elastic Stack Elasticsearch
1

We are using synthetic _source and date fields with format defined as epoch_millis. Our expectation would be to get such fields as long number but we get them as strings.
As we understand it might be necessary when dates are being stored in most other formats, in this one it is odd.

Currently we got following from the search API:

{
  ...
  "hits" : {
    ...
    "hits" : [
      {
        ...
        "_source" : {
          "_created" : "1608193593998",
          ...
        }
      },
      ...
    ]
  }
}

for mapping:

{
  "index_name" : {
    "mappings" : {
      "_source" : {
        "mode" : "synthetic"
      },
      "properties" : {
        "_created" : {
          "type" : "date",
          "format" : "epoch_millis"
        },
        ...
      }
    }
  }
}

Is there a way to configure mapping the way that we get our date fields in epoch_millis format as long numbers even with synthetic _source please?

Expected search API output:

{
  ...
  "hits" : {
    ...
    "hits" : [
      {
        ...
        "_source" : {
          "_created" : 1608193593998,
          ...
        }
      },
      ...
    ]
  }
}