Date format switches when deserializing Json

Question

I have a background process that fills a datatable. It works by retrieving a small amount of rows from an API in Json format and deserializing them to a datatable (loop). These rows are then added to another datatable which will contain all rows after the API no longer has results.

Now what happens is that after about 3-4 calls to the API, the JsonConvert in the client deserializes rows into a different date format than the rows that came before. Which then gives an error when trying to add them to the main datatable.

Error: String was not recognized as a valid DateTime.Couldn't store <12/20/2019 00:00:00> in deliverydate Column. Expected type is DateTime."

I checked the database formats and the Json formats from the api and these are all in yyyy-MM-dd. However after those 3-4 calls JsonConvert suddenly uses MM/dd/yyyy.

I've tried adding JsonSerializerSettings to the converter, changing windows regional settings and forcing a CultureInfo however none of this worked.

allData = JsonConvert.DeserializeObject<DataTable>(jsonString);                        

foreach (DataRow dr in allData.Rows)
{
    tempTable.Rows.Add(dr.ItemArray);
}

The 'jsonString' variable contains the Json from the api. The tempTable is the datatable containing all the rows.

Edit: example of the jsonString:

 {
    "id": 2453,
    "shoppingcartid": 2453,
    "creationdate": "2019-08-06T00:00:00",
    "ordernumber": 2738,
    "ordernumberStr": "CS358405",
    "invoicenumberStr": "10724761",        
    "customername": "ITT",
    "customerid": 1182,
    "managerid": 5186,
    "enduser": null,
    "enduserid": null,
    "location": null,
    "locationid": null,
    "state": "Open",
    "stateid": 1,
    "type1": "T4 21.9 LZ",
    "type1specification": 2,
    "vehicle": "NVT",
    "vehiclespec": 12,
    "deliverydate": "2019-12-20T00:00:00",
    "start": "2019-12-09T00:00:00",
    "end": "2019-12-20T00:00:00",
    "isplanned": false,
    "desireddeliverydate": "2019-12-20T00:00:00"
}