Bricks Builder: invert the default behaviour of the dropdown inside the Nav element (opened when the menu is clicked).

bricks-mobile-dropdow-open-nav-element.html

/* Nav element: change based on the breakpoint for the mobile menu. */
/* Note: change the `max-width` value to reflect the Nav element's breakpoint */
@media ( max-width: 768px ) {
    .display-sub-menu {
        display: block !important;
    }
}

<script>
/**
 * Nav element.
 * Invert the default behaviour of the dropdown inside the element.
 * Toggles the submenu with a single CSS class (.display-sub-menu).
 */

// Run the function once the window has fully loaded
window.onload = function() {
    // Select all elements with the class 'sub-menu'
    var subMenus = document.querySelectorAll('.sub-menu');

    // Loop through each '.sub-menu' element
    subMenus.forEach(function(subMenu) {
        // Add the 'display-sub-menu' class to each '.sub-menu' element
        subMenu.classList.add('display-sub-menu');
    });

    // Select all buttons within elements with the class '.brx-submenu-toggle'
    var buttons = document.querySelectorAll('.bricks-menu-item .brx-submenu-toggle button');

    // Loop through each button
    buttons.forEach(function(button) {
        // Add a 'click' event listener to each button
        button.addEventListener('click', function(event) {
            // Stop the event from bubbling up the DOM tree
            event.stopPropagation();

            // Try to select the '.sub-menu' element that is a sibling of the button
            var subMenu = event.target.closest('.sub-menu');
            // If the '.sub-menu' element exists
            if (subMenu !== null) {
                // Toggle the 'display-sub-menu' class on the '.sub-menu' element
                subMenu.classList.toggle('display-sub-menu');
            } else {
                // If the '.sub-menu' element does not exist, it means the button has been clicked again
                // Find the '.sub-menu' element that is a sibling of the button's closest '.brx-submenu-toggle' ancestor
                subMenu = event.target.closest('.brx-submenu-toggle').nextElementSibling;
                // If the '.sub-menu' element exists and matches the '.sub-menu' selector
                if (subMenu !== null && subMenu.matches('.sub-menu')) {
                    // Toggle the 'display-sub-menu' class on the '.sub-menu' element
                    subMenu.classList.toggle('display-sub-menu');
                }
            }
        });
    });
};
</script>

Don't forget to add the following CSS:

/* Nav element: change based on the breakpoint for the mobile menu. */
/* Note: change the `max-width` value to reflect the Nav element's breakpoint */
@media ( max-width: 768px ) {
    .display-sub-menu {
        display: block !important;
    }
}