Switch to the mobile device, select the menu element - if you don’t find it right away, click into an element in the menu banner preset and use the arrow keys until you find the menu element - keep an eye on the breadcrumb at the bottom of the website builder
Then go to Style > Element > Animation and select “Show/Hide”. Set Initial to “Element hidden”, Duration and Delay to 0 and Trigger to “Element scrolled into view” - save the page and that’s it.
Just if you want to reverse my steps: Change the Animation from “Show/Hide” to “Don’t animate”.