I’m still thinking there’s some sort of caching going on. Server caching? Cloudflare?
Your site is still requesting this file:
<link rel='stylesheet' id='generate-mobile-header-css' href='https://yourwebsite.com/wp-content/plugins/gp-premium/menu-plus/functions/css/mobile-header.min.css?ver=1.7.2' type='text/css' media='all' />
mobile-header.min.css no longer exists in GPP 1.8.0, and you’ll see the version is still set to 1.7.2. When you try to open that URL, the file doesn’t exist on the server (as it shouldn’t if you’re using 1.8.0).
So something is caching the output of your page.