Corrupt output using XmlWriter. Am I doing something wrong?

I can reproduce my problem using this straightforward C# console application code:

        static void Main( string[] args )
        {
            StringBuilder xmlsb = new StringBuilder();

            using ( XmlWriter xw = XmlWriter.Create( xmlsb ) )
            {
                xw.WriteStartDocument( true );
                xw.WriteStartElement( "MyDocument" );

                for ( int fi = 0; fi < 50; ++fi )
                {
                    xw.WriteStartElement( "item" );

                    for ( int indx = 10; indx < 55; ++indx )
                    {
                        string attrName = "Attribute.Name." + indx;
                        string attrValue = "dummy123456789";
                        xw.WriteAttributeString( attrName, attrValue );
                    }
                    xw.WriteEndElement();
                }

                xw.WriteEndElement();
                xw.WriteEndDocument();
            }

            string strXML = xmlsb.ToString();
            strXML = strXML;
        }

The values are chosen to make it easier for demonstration purposes to spot the corruption in the resultant xml.

I've only seen any corruption when there's quite a bit of output - those values and strings seem to reproduce it for me.

To test this yourself, put a breakpoint on the strXML = strXML line and run the program.

When it breaks, examine the string in the debugger text visualizer, copy the entire string to the clipboard and paste it into a blank XML document in VS (have VS set to format the XML to put each attribute on a separate line), then scroll down the document and look for anywhere the otherwise tidy alignment goes awry.

I inevitably see something like this around half way through the XML:

        Attribute.Name.21="dummy123456789"
        Attribute.Name.22="dummy123456789"
        Attribute.Name.23="dummy...y123456789"
        Attribute.Name.40="dummy123456789"
        Attribute.Name.41="dummy123456789"

The corruption can be in the attribute or value.

Have I got something wrong in my code that I just can't spot?