c-sidebar-nav-item c-sidebar-nav-dropdown on mobile,when clicked the whole side menu is collapsed

On mobile view, when the drop down menu item is clicked, its items should appear for the user directly, but what really happens now is that on click, the whole side menu is collapsed.
Please try it on demo: https://coreui.io/demo/free/3.4.0 
browse the link on mobile view, then click Base menu item for example, and you will know what I mean.
Thank you,