Well figured it out by myself. z-index was set on the wrong element. Add a new class to the container_class property:
<?php wp_nav_menu(
array(
'theme_location' => 'header-menu-left',
'dept' => 2,
'menu_class' => 'navbar-nav justify-content-end',
'container_class' => 'collapse navbar-collapse YOURCLASS',
'container_id' => 'navbarNav',
'fallback_cb' => 'WP_Bootstrap_Navwalker::fallback',
'walker' => new WP_Bootstrap_Navwalker(),
)
); ?>
and change the css:
.YOURCLASS {
padding: 0px;
top: 0;
z-index: 10;
}
Note: Before those changes the z-index was set on the navbar itself. To get it work properly you have to set z-index on the container within the navbar. Just to clarify this.