Menu disappears when I scroll on a mobile device

I have a problem with my mobile site. on iPhone 8 when i go to my site the menu is accessed by the button with the three lines in it. I press this button and the menu appears. I have to scroll down to see the rest of the options in the menu but the menu disappears. I try to click on the menu button again and it wont show the menu. I have tried disabling all the plugins and this did not help. I am using the iPhone 8 with Safari as the browser.

The page I need help with: [log in to see the link]