Following your message, i have checked your site in IE on window 7 and Internet Explorer Edge on window 10 like exactly your image and see the header of your site still display well.
I can not help you fix it if i do not see the problem.
Also the new header is a trick. theme will load old theme style first then my custom css to apply new header style. In order to hide the loading step, you can use Preload feature in Traveler Settings > Preload Option.