Incident auto closure did not close incidents after not being updated for the defined number of days.
The incident autoclose queries incidents were incident_state is Resolved. In this case, the state is Resolved; however, the incident_state is not. In an out of box instance, the state and incident_state are sync'ed via two business rules ('Copy State to Incident State' and 'Copy Incident State to State'). Deletion of the 'Copy State to Incident State' cause the incidents to no longer be sync'ed.
Ensure the incident_state value is always sync'ed to the state value to ensure the incident autoclose process is able to accurately identify the the appropriate records.