Saving OrderGroup TaxAmount not working

I'm calling the IOrderRepository method Save that takes an IOrderGroup, but the TaxTotal amount I update is not persisting to the database.

Here is a code snippet (pulled from many methods to be concise)...

var purchaseOrder = _orderRepository.Load(orderId);
var order = purchaseOrder as OrderGroup;
 if (order != null && taxResult?.TotalTaxes != null) {    order.TaxTotal = taxResult.TotalTaxes.MoneyValue; // I see this get set to a dollar amount
 }
if (order != null) {    var orderGroup = order as OrderGroup;    if (orderGroup == null) {        result = _orderRepository.Save(order);    }    else {        // at this point, I still see the dollar amount set above        result = _orderRepository.Save(orderGroup); // this is the code path that gets executed    }
}

End result is a TaxTotal of 0.0 in OrderGroup table.  This is driving me crazy.  Any help will be appreciated.