Query for wells with spud date greater than certain value not returning expected result

When running the query below:
{
   "kind": "opendes:osdu:well-master:0.2.1",
   "query": "data.SpudDate: > 1999-04-20",
   "sort": {
        "field": ["data.SpudDate"],
        "order": ["ASC"]
    },
    "returnedFields": ["data.SpudDate", "data.UWI"],
    "limit": 5
}

The following results are returned:
{
    "results": [
        {
            "data": {
                "UWI": 4291,
                "SpudDate": "1892-04-07T00:00:00+0000"
            }
        },
        {
            "data": {
                "UWI": 4310,
                "SpudDate": "1898-04-25T00:00:00+0000"
            }
        },
        {
            "data": {
                "UWI": 4326,
                "SpudDate": "1901-04-17T00:00:00+0000"
            }
        },
        {
            "data": {
                "UWI": 4328,
                "SpudDate": "1902-04-01T00:00:00+0000"
            }
        },
        {
            "data": {
                "UWI": 1161,
                "SpudDate": "1905-04-10T00:00:00+0000"
            }
        }
    ],
    "aggregations": null,
    "totalCount": 953
}

I was expecting to only get well with spud dates greater than 1999-04-20. Is this a valid query?