Divi is a great theme, but some features do not work the way they are supposed to. For example, the secondary menu which appears on the top of the page, is added to the end of the mobile menu.

Divi Secondary Menu on Desktop
Divi Secondary Menu on Mobile

This might not be what you want. You might want to hide the secondary menu on the mobile menu. To hide the secondary menu on mobiles and tablets just add the following CSS to the Additional CSS section of the Theme Customizer:

/* hide secondary menu on mobile */
mobile_menu li:not([id]) { display:none !important;

This should do the trick.