Edit (14-Feb-2021): This issue has been resolved in more recent versions of the Pen theme. Refer to the forum post Pen Theme compatibility with iOS for details.
There appears to be an issue with the WordPress Pen theme, specifically the hamburger menu, and earlier versions of iPhone/iOS. The site referred to in the browser is readymcgetty.com.au. Tested using both Google Chrome and Apple Safari browsers.
The hamburger menu works fine with an iPhone 11 Pro Max running iOS 14.2.
The menu flashes briefly before disappearing on an iPhone 5s running iOS 12.5.
Similarly with an iPhone 6s running iOS 13.7.