Issue with dynamic_date_formats for string with epoch_millis

Hi, I'm trying to understand why "dynamic_date_formats":"epoch_millis" doesn't work.
The PUT my_index_one/_doc/1 below doesn't return. It seems to hang in Kibana Dev Console.
I expect it to return and the foo field to be mapped as a date.

PUT _template/template_1
{
    "order":0,
    "index_patterns":[
        "my_*"
    ],
    "settings":{
        "index":{
            "number_of_shards":"1",
            "number_of_replicas":"0"
        }
    },
    "mappings":{
        "date_detection":true,
        "dynamic_date_formats":"epoch_millis"
    }    
}
PUT my_index_one/_doc/1
{
  "foo": "1574420642643"
}
GET my_index_one/_mapping
GET my_index_one/_doc/1