Safari, iOS 11.2.5 and display of fixed menu bar

I am using a transparent and sticky header on this development site http://test.corvetteclubsantabarbara.com. On MS Edge Windows box and Google Chrome/Safari on an iPad, the menu bar works perfectly and stays fixed while scrolling or navigating down the one page site. However, on my iPhone 7+ using the same iOS 11.2.5 as my iPad, the sticky header vanishes. This same problem exists for the Google Chrome mobile browser on my iPhone as well. Comet Cache Pro is used on this site and I disabled Client Side Cache so that’s not a source of the problem. Local cache is purged consistently on all devices but the problem still remains. Any suggestions would be appreciated.