How to edit custom Line Manager role to not modify direct reports' line manager

We have had a couple instances now where Line Managers were able to remove an employee's existing line manager and reassign the employee to another line manager. We have a custom Line Manager role and have honed in on the Manage Direct Reports duty roles since it appears the individuals who have made the changes have gone through managing direct reports. These duty roles have been set up identically to the delivered Line Manager role which we've been told does not have this issue. Any advice or assistance in finding a solution is greatly appreciated.