Hello,

The manager in "Departements form" will be the default value into "employee form" when you will set a department ... (on_change_department_id function)

But the manager id will be saved into the Employee table. 

To get what you explains... you need to replace this "manager field (parent_id)" with a related to department_id.manager_id

Update : 

In department form, you define the default manager for this department.

So when you are in employee form, when you set the department, by default the manager field is fill in with the default value. But you are allowed to edit this one, that just a default value... So, 2 employees from the same department can have 2 distincts managers !

It's not a bug, it's the behavior that Odoo wants...

If your idea, is to define only the department to an employee and only show for information the manager of the current deparment, you need to use a "related field" to keep the relation between department/manager. With this solution, if you change the manager in employee form, your are changing the default manager for this department...

JKE