There are two menu instances on your page - <div id="primary-nav"> and <nav id="mobile-menu"> . In the HTML source of the page, they look similar, but the debug panel shows that the problematic menu item is wrapped in two links at once:
<a href="#" role="menuitem" aria-haspopup="true" tabindex="-1" class="slicknav_item slicknav_row" style=""> <a href="#" tabindex="-1">Категория купонов</a> <span class="slicknav_arrow">▼</span> </a>
If you leave only one link, the list of sub-items begins to unfold as it should.
<a href="#" role="menuitem" aria-haspopup="true" tabindex="-1" class="slicknav_item slicknav_row"> Категория купонов <span class="slicknav_arrow">▼</span> </a>
To start, try to leave this item in the mobile menu completely without a link. If one of your scripts adds the missing link, it will work. If you do not add, then you need to dig into the scripts and carefully follow their hands.