Secondary menu in GP 3.0

Hi,

I often use the secondary menu for the language switcher. On mobile it looks weird to have two menus, so I always use this css to change the menu toggle into a displayed menu:

/* DISABLE SECONDARY MENU TOGGLE */
.secondary-navigation .menu-toggle {
display: none;
}

@media(max-width: 768px) {
.secondary-navigation {
text-align: center !important;
}
.secondary-navigation ul {
display: block;
}
.secondary-navigation .sf-menu>li {
float: none;
display: inline-block !important;
}
}
/* END SECONDARY MENU TOGGLE */

That works perfectly on all GP sites, even on GP 3 testsites, but

NOT when I change the structure to Flexbox. The upper part of the css does make the menu toggle disappear, but it just hides the entire menu. What do I need to change?