Steps to reproduce the issue

I have multiple user groups in a way like this:

• Departments
  -- Dep1
  -- Dep2
  -- Dep3

And I have a menu for each department.
For obvious reasons, they should not be able to edit the other departments menu in the Backend.

As suggested in the docs, I have set the "menu edit permission" of the user group "Departments" to disabled/inherited (can not edit).

Then I set the permission of each menu to allow editing by one Dep.

Expected result

Each department can edit its own menu.

Actual result

nobody can edit menus.

System information (as much as possible)

3.9

Additional comments

Can someone suggest a way to solve this?
Or it even possible to restrict access to some menus in the Backend?