SslStream发送UTF-8多字节URL编码时%被转义的原因问询

我尝试通过SslStream和StreamWriter发送包含特殊字符的URL GET请求，代码如下：

var req = new HttpRequestMessage( HttpMethod.Get, m_client.BaseAddress );
m_sessionClient.AddSessionInfoTo( req );

var uri = req.RequestUri;

var client = new TcpClient( uri.Host, uri.Port );
Stream stream = client.GetStream();
var ssl = new SslStream( stream );

ssl.AuthenticateAsClient( uri.Host );

var writer = new StreamWriter(
                ssl,
                new UTF8Encoding( encoderShouldEmitUTF8Identifier: false )
);

await writer.WriteAsync( $"{req.Method.Method} " );
await writer.FlushAsync();

var path = "/echo/%C9%52";
var rawPathAndQuery = Encoding.UTF8.GetBytes( path );
await ssl.WriteAsync( rawPathAndQuery, 0, rawPathAndQuery.Length );

await writer.WriteAsync( " HTTP/1.1\r\nHost: " );
await writer.WriteAsync( req.RequestUri.Host );
await writer.WriteAsync( "\r\n" );
await writer.WriteAsync( "\r\n" );
await writer.FlushAsync();

这段代码预期发送的请求路径是/echo/%C9%52（对应字符ÉR），但通过Wireshark捕获到的实际请求却是GET /echo/%25C9R HTTP/1.1——%C9被转义成了%25C9，相当于%字符本身被URL编码了。

这个问题仅出现在多字节UTF-8场景（即编码值≥%80的字符）中。我想知道导致该行为的原因，不需要解决方案。我查过StreamWriter和SslStream的文档，没有找到任何关于UTF-8转义的相关说明，而且在浏览器中直接访问echo/%C9是可以正常工作的。

内容的提问来源于stack exchange，提问作者AnaCoda