Customer is using PAC file (Proxy Auto-Configuration). This could be hosted in Proxy itself or on a local server. Any changes made in this PAC file is not reflecting when using Internet Explorer 11 browser. Changes are showing up correctly when using other browsers.
The issue is to due to the default behavior of IE11 to cache the PAC file it received. This makes it difficult to test PAC file changes as IE11 will still prefer to use the cached entries.
Resolution for this is to disabled PAC file caching in IE11. Instructions to this is available at MicroSoft article 271361. This will make modifications in the registry so that browser IE11 will not cache PAC files.