vbege
Because once a ticket is closed it's considered done, faneto, resolved. Since it's resolved no one needs to be assigned, it should no longer have a due date, and it is no longer overdue. You simply can't view this information via searches/queues; only via the Dashboard Statistics. You can run manual SQL queries within your database to calculate such statistics but you'll soon realize you'll be basically copying what the dashboard already does.
And no, this is always how osTicket has functioned (as far as I can remember - and I joined the team circa 1.9.x or 1.10.x)
Cheers.