I have a saved search for completed tasks by my direct reports. If there is a task with no assignee that gets completed then it is omitted from these reports since they are not assigned. Would be great if there were a filter for “Completed By”.
That would also fix any issues with a person other than the assignee completing a task so they could get proper credit.
Obviously this is all fixed by properly assigning tasks but when you are creating subtasks within a project, it’s very easy to create a bunch of tasks to be worked on and not think to assign them.