Hi Tino,
It’s a problem with the menu script, and how it clones the DOM nodes. Menus are usually part of themes, unless you have a separate menu plugin installed. Unfortunately many theme developers use this “clone” method to display menu items. It’s a very comfortable method, but basically prevents placing most 3rd party javascript powered item into the menu (like sliders, control forms, live searches, drop-downs etc..). I believe they don’t expect the customer to place anything fancy there, so I don’t blame them for this.
You can reach out to the theme developer, but I highly doubt he will be able to do anything with this, as it would require him to completely re-design the menu to not use javascript for cloning the menu elements.
A good menu example is the twentysixteen default theme menu, which simply moves the elements with CSS styles, without any javascript involved.