Welcome, @Connie_Chen1,
What you point out is a deficiency, I’m sorry to say.
You can vote for this feature here:
Meanwhile, the best you can do is find the tasks assigned to you that are completed in a saved search to catch a lot of them, but the rest you’d have to search for completed tasks, not assigned to you, within the last period, and then look one-by-one at the stories (audit trail) to see who completed each one.
Larry