Module's control panel sub-navigation is not working properly
This is working for me.
Module.php
        Event::on(Cp::class, Cp::EVENT_REGISTER_CP_NAV_ITEMS, function (RegisterCpNavItemsEvent $event) {
            $event-&gt;navItems[] = [
                'url' =&gt; 'modules/video',
                'label' =&gt; 'Video-Converter',
                'icon' =&gt; '@modules/icon-mask.svg',

                'subnav' =&gt; [
                    'inhex' =&gt; [
                        'url' =&gt; 'modules/video/index',
                        'label' =&gt; 'Project-Video',
                    ],
                    'old' =&gt; [
                        'url' =&gt; 'modules/video/old',
                        'label' =&gt; 'Old Projects',
                    ],
                ]
            ];
        });

And in the templates I am using e.g.
{% extends '_layouts/cp' %}

{% set title = 'Old Projects' %}
{% set selectedSubnavItem = 'old' %}