How to hide menus properly?

Sorry, I didn’t quite understand that. What does “submenus on the same level as hidden menus” mean, and what do arrow icons have to do with it? The screenshot just shows a small number of menus, likely using a custom admin theme. What is it intended to demonstrate?

Could you perhaps describe the problem in a different way?

I added plugin menus in settings. These menus had a sub-level.

Unfortunately many of these sublevels do not have an icon to their left. So it is not clear which group of menus they belong to. https://postimg.cc/Cz4pgCX7
https://postimg.cc/K42ZhY4Z

I understand that the icons on the left are not added by you. But if your plugin could add an icon on their left, I’d appreciate it. Or show the sublayer as it was before.

I’m still not entirely sure what you’re describing, but, as a guess, are you saying that you tried to create a nested submenu inside another submenu? That is, did you take a menu hierarchy like “Settings -> A” and try to add another level, like “Settings -> A -> B”?

WordPress currently only supports two admin menu levels: top-level menus and submenu items. By default, it’s impossible to add more levels. The free version of Admin Menu Editor cannot add more levels. The Pro version can add one more level, but it’s an experimental feature that might not be compatible with some plugins/themes.

As for those arrow icons in your new screenshots, I’m pretty sure that’s not a standard WordPress feature. They were probably added by another plugin that I’m not familiar with. I don’t know how they work, or how to add/remove them.