I do more testes to clarify the issue.
Here is the code in both ASP and ASP.Net
<% Response.Write(Request("kw")) %>
Here are the request for ASP and ASP.Net
http://localhost/test.asp?kw=中文測試
http://localhost/test.aspx?kw=中文測試
Here are the results:
1. ASP: ASP doesn't have display issue, it can display Chinese Characters correntcly

2. ASP.Net
By default setting (.Net Globalization = UTF8, App Pool in Integrated Mode) ASP.Net can not display Chinese characters:

After changing App Pool from Integrated to classic and .Net Globalization requests encoding from UTF-8 to big5, the Chinese characters display correctly

But in both cases neigher correctly log cs-uri-query in log files

Conclusion:
There are some encoding issue in ASP.Net in integrated app pool and IIS Log. Hope these testes can help IIS team to figure out what goes wrong.