At this time, if your menu has only one dropdown, it will unfortunately not work on mobile. The best course of action here would be to create and include in the menu at least one more dropdown, which you can then hide, and this will resolve the issue of the first dropdown not displaying.
Here’s how you can go about adding one more dropdown and hiding it from the menu, thus effectively enabling the one dropdown to appear on mobile:
We are planning to roll out a fix for this issue in future updates.