it makes sense, newer versions of browsers are not recognized until they get updated browser definition files in the .NET framework updates, unrecognized web browsers are treated as downlevel browsers. actually I'm surprised you are not having problems logging in with IE 11 because there are known issues with IE 11 login when the server does not have the updated browser definition files.
you may also want to check the web browser settings in IE 11 and make sure it is not using compatibility mode for the site, look under Tools > Compatibility settings
you may also want to try different skins and see if you get the same result, there could be a meta tag in the layout.master that forces IE compatibility mode for example and you don't want that