Bundling not working in MVC5 when I turn on release mode

This is the default behavior.

Bundling and minification is enabled or disabled by setting the value of the debug attribute in the compilation Element in the Web.config file.

<system.web>
    <compilation debug="true" />
    <!-- Lines removed for clarity. -->
</system.web>

To enable bundling and minification, set the debug value to “false”. You can override the Web.config setting with the EnableOptimizations property on the BundleTable class. The following code enables bundling and minification and overrides any setting in the Web.config file.

public static void RegisterBundles(BundleCollection bundles)
{
    bundles.Add(new ScriptBundle("~/bundles/jquery").Include(
                 "~/Scripts/jquery-{version}.js"));

    // Code removed for clarity.
    BundleTable.EnableOptimizations = true;
}

http://www.asp.net/mvc/overview/performance/bundling-and-minification