If you find that your submenu link items disappear when scrolling down, it is possible that this is actually the result of some counter-intuitive behavior of Divi's own settings.
In "Divi > Theme Customizer > Header & Navigation" the "Fixed Navigation Settings > Primary Menu Link Color" option overrides the "Primary Menu Bar > Dropdown Menu Text Color" setting.
If "Primary Menu Bar > Dropdown Menu Background Color" matches the "Fixed Navigation Settings > Primary Menu Link Color", the submenu links will have the same color as the submenu background and won't be visible when scrolling down activates the fixed header.
To fix this, either choose non-conflicting colors for these settings, or you can manually override the submenu link color with:
Related Post: Adding CSS to the Divi Theme