What Pega version are you using? I would start by looking at the work object history for the objects where the issue occurred and see if there are any similarities. Then see if you can reproduce the issue by following the same path shown in the history.
You shouldn't have any "Resolved Completed" work objects on a user's work list. A work list is just supposed to handle assignments for open work objects. It looks like the work object has been resolved but the assignment is remaining. This is a missed configuration, you will need a ticket on your flow to automatically remove the assignment when somebody else is closing the case.