The XPath number() function in XPathNavigator.Evaluate method returing wrong value

Question

I using the excellent Evaluate method of the XPathNavigator object. Please look at this block of code (simplified):

public class Test
{
    public decimal? Number5 { get; set; }
}

var obj = new Test()
{
    Number5 = null   
};

var xml = new XmlDocument();
xml.LoadXml(ToXMLString(obj));
XPathNavigator nav = xml.CreateNavigator();

var result5 = nav.Evaluate("number(//Number5)");

The result5 should contain "NaN" but it contains "¤¤¤"!

This has worked for years on our developer Machines (Windows 7 / .NET FM 4.6) and old servers (Windows Server 2008 / .NET FM 4.5).

But not on our new Windows Server 2016 .NET FM 4.6. It is a Swedish OS and a Swedish .NET Framework.

Please, please help!