If a manager sees “unknown manager” instead of a name in an employee’s past performance review, it usually means the review was written by a manager who has since been terminated or the manager cannot access to the previous manger's data for other reason — permissions are hiding their identity.
If the previous manager’s lifecycle status is Terminated, and managers don’t have permission to view terminated employees, Bob replaces the name with “unknown manager.”
The important part to know:
if the manager have permission to "See reviews of direct and indirect reports" within the Managers permission group under People's data > Reviews. The review itself is still visible. Only the former manager’s name is masked.
