Magento 2: Admin panel 3 level menu structure not working

After research a lot. I found that it is a not default Magento Bug But it is a feature!

Actually, you can not see parent title because you have a single group.

Let's understand with an example :

In your custom module, you have created Grand menu > Parent menu > Child menu

So in above Parent menu is a single group (You have only a single parent menu).

But if you create another group menu under Grand menu like : Grand menu > Parent menu 2 > Child menu

So here you have created 2 Parent menu (1) is Parent menu & (2) Parent menu 2 so basically it is 2 groups. if menu group is more then 1 group then Parent menu will display. It will not display if you have only single group menu.

Magento Core Example :

Go to Admin > Content there is 2 groups Elements and Design

So it's parent menu is displaying. check below screen shoot.

But Go to Admin > Sales there is only single group Sales so parent menu is not displaying.

Conclusion : If you want to display parent menu then you have to create 2 groups in the menu. Like above screenshot.

• Issue is created in Git hub
• Issue rejected Reason (Solution)

Let me know if you have any query/concern in the above explanation. I hope it helps!

Honestly, I didn't give the proper answer to your question. But found a way to show the parent menu. Actually, Magento is hiding First "parent" menu via CSS. So you need to add another parent menu and will display your previous parent menu. Here is my code:

    <?xml version="1.0"?> <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Backend:etc/menu.xsd">
        <menu>
            <add id="Vendor_Module::jsutariya" title="Grand Parent" module="Vendor_Module" sortOrder="100" resource="Vendor_Module::jsutariya" />
            <add id="Vendor_Module::jsutariya_listgallery" title="Parent" module="Vendor_Module" sortOrder="10" resource="Vendor_Module::listgallery" parent="Vendor_Module::jsutariya" />
            <add id="Vendor_Module::jsutariya_settings" title="Child" module="Vendor_Module"
                 sortOrder="40" action="adminhtml/system_config/edit/section/list_gallery"
                 resource="Vendor_Module::settings" parent="Vendor_Module::jsutariya_listgallery"/>
            <add id="Vendor_Module::jsutariya_listgallery1" title="Another Parent" module="Vendor_Module" sortOrder="20" resource="Vendor_Module::listgallery" parent="Vendor_Module::jsutariya" />
        </menu> 
</config>

The main culprit is the following CSS and I don't know why its added in the first place by Magento

.admin__menu .level-0 > .submenu > ul > .level-1:only-of-type > .submenu-group-title {
    display: none;
}

You have to need change _menu.less for fix the child menu issue actually Admin Navigation Menu labels hide in css.

You can change below _menu.less file path and comment code which is already display:none.

File path:

magento/vendor/magento/theme-adminhtml-backend/Magento_Backend/web/css/source/module/_menu.less

//  This part hides Submenu Group Titles only for menus with single groups.
    /*.submenu .column:only-of-type .submenu-group-title,
    .level-0 > .submenu > ul > .level-1:only-of-type > .submenu-group-title {
        display: none;
    }*/

For more Details refer below links:

https://github.com/magento/magento2/issues/13707

https://github.com/magento/magento2/commit/d91e96b529e70dcec55d3b5336b471791dc42d54