Hi James,
That means, that the search stylesheets are not properly loaded on the front-end or they are completely missing. The most common causes are:
– The website cache is not cleared, make sure to clear all layers of cache – including minification, asset and page cache
– If you have a CDN provider, the CDN cache should be cleared as well
– If still no luck, then the wp_footer() statement could be missing from the theme footer.php file, although this is a very rare theme related mistake
I am currently seeing the search bar properly styled in the header, so I guess it was only a cache related problem?