Targeting specific nav menus in a function

I would like a nav menu with icons and description in a mega-menu. So I use ACF to add icons and description, and wp_nav_menu_objects to inject them into the menu.
The trouble is, it injects them into every menu. I only want to target some menus. I tried registering some locations and targetting them, but the location is always empty:

function ds_custom_menu_location() {
      'quick-menu' => __( 'Front Page Help Menu' ),
      'mega' => __( 'Mega Menu' )
add_action( 'after_setup_theme', 'ds_custom_menu_location' );

function ds_wp_nav_menu_objects( $items, $args ) {
	if($args->theme_location !== "mega") return $items;
	foreach( $items as &$item ) {
        $icon = get_field('mmenu_icon', $item);
		$menudesc = get_field('category_menu_desc', $item);
        if( $icon && $menudesc ) {
            $item->title = '<div class="mmenu-icon bricks-color-dark"><i class="fa fa-'.$icon.'"></i></div><div class="mmenu-texts">' . $item->title;
            $item->title .= '<div class="mmenu-desc bricks-color-dark">' . $menudesc . '</div></div>';        
			return $items;
add_filter('wp_nav_menu_objects', 'ds_wp_nav_menu_objects', 10, 2);

I only half understand what I’m doing, and wondered if it’s necessary to instantiate the menu with its location? Which I can’t do in native Bricks? Or if I’m missing something.
Why is $args->theme_location always empty?
Thanks in advance.