There are two separate due dates.
When someone opens a ticket they can specify a duedate.
(They do not have to) This is stored in ost_ticket.duedate.
This due date is User or Agent selected and is not tied to the SLA.
When someone/system applies an SLA to a ticket. It calculates the SLA's due date.
This is stored in ost_ticket.est_duedate.
'SLA due date' is NOT a tickets 'Due date'. SLA Due Date would be when your SLA for that ticket has expired.
It can expire and you can still have a (ticket) duedate in the future as they are not related.
It sounds to me like you are confusing the two different items.