Why does the submenu collapse on the phone in Safari?

Question

css

Good afternoon,

I have a vertical menu on the site, when you click on the menu item down, a submenu of several items expands. On the iPad and phone in all browsers, everything is displayed correctly, except for Safari. When you open a site on an iPad (phone), I click on a menu item, my submenu opens, but in order to view it completely, I need to scroll down the page a little. I just start scrolling, and immediately the submenu collapses. And by the way, this behavior happens with some other elements on the site that unfold when clicked.

But I noticed that when you start scrolling the page in the browser and the address bar in it disappears, this folding occurs.

Unfortunately, I can not give a link to the site (everything is too confidential :)).