Return Json data from POST action in Core/knockout

I am working with the Knockout JavaScript library in .NET Core. I have the following POST action that successfully receives and handles the Json data from my view. What I cannot get to work is the return to the client:

1) Ideally I want it to return the updated back to the view. However there seems to be a fault with this .NET Core which means the mapping does not work. None of the following work:

        return Json(JsonConvert.SerializeObject(salesOrderViewModel));
        return Json(salesOrderViewModel);
        return Json( new { salesOrderViewModel });

2) But no other return seems to have any effect either. For example:

        //return NotFound();
        //return RedirectToAction("Index");
        //return View("Index");
        //return DoSomething();
    }

    private IActionResult DoSomething()
    {
        return RedirectToAction("Index", "Bird");
    }

Below I have pasted the View, the client side viewmodel, and the full controller action. Can anyone give me any insight into why I cannot return anything from my controller action?

View:

@model Birder2.ViewModels.SalesOrderViewModel
@using Newtonsoft.Json
@{
    ViewBag.Title = "Sales Order Details";
}
@{
string data = JsonConvert.SerializeObject(Model);
}
@section scripts{
<script src="~/js/knockout-3.4.2.js"></script>
<script src="~/js/knockout.mapping-latest.js"></script>
<script src="~/js/salesorderviewmodel.js"></script>
<script type="text/javascript">
    var salesOrderViewModel = new SalesOrderViewModel(@Html.Raw(data));
    ko.applyBindings(salesOrderViewModel);
</script>
}
<p data-bind="text: MessageToClient"></p>
<div>
    <div>
        <label>Customer Name:</label>
        <input data-bind="value: CustomerName" />
    </div>
       <div>
        <label>P.O. Number:</label>
        <input data-bind="value: PONumber" />
   </div>
</div>
<p><button data-bind="click: save">Save</button></p>

The client-side view model:

SalesOrderViewModel = function (data) {
    var self = this;
    ko.mapping.fromJS(data, {}, self);

    self.save = function () {
        $.ajax({
            url: "/SalesOrders/Save/",
            type: "POST",
            data: ko.toJSON(self),
            headers:
            {
                "content-type": "application/json; charset=utf-8"
            },
            success: function (data) {
                if (data.salesOrderViewModel)
                    ko.mapping.fromJS(data.salesOrderViewModel, {}, self);
            },
        });
    };
};

The full controller action is here:

    //[HttpPost]
    //public JsonResult Save([FromBody]SalesOrderViewModel salesOrderViewModel)
    [HttpPost]
    public IActionResult Save([FromBody]SalesOrderViewModel salesOrderViewModel)
    { 
        SalesOrder salesOrder = new SalesOrder();
        salesOrder.CustomerName = salesOrderViewModel.CustomerName;
        salesOrder.PONumber = salesOrderViewModel.PONumber;

        _context.SalesOrders.Add(salesOrder);
        _context.SaveChanges();

        salesOrderViewModel.MessageToClient = string.Format("{0}’s sales order has been added to the database.", salesOrder.CustomerName);

        return Json(JsonConvert.SerializeObject(salesOrderViewModel));
        //return Json(salesOrderViewModel);
        //return Json( new { salesOrderViewModel });

        //return NotFound();
        //return RedirectToAction("Index");
        //return View("Index");
        //return DoSomething();
    }

The data are successfully bound and the database is updated). Nothing is returned to the client.

Made changes to View such that we bind knockout properly and also use proper objects in success method.