SLA is not pausing as expected.
Custom UI action.
The behavior seen is due to custom UI action 'Close Task'. The script running in this is causing the update to task_sla records to fail.
Using these OOB ui actions, the task sla record is updating to Paused as expected. This can also been seen by setting the sc_task record to closed complete from the RITM related list 'Catalog Tasks'. The only time this is failing to update the task sla is when this custom ui action being used.
From reviewing this UI action it appears the RITM is getting updated with information from the sc_task record when this is pressed. One example of a solution here would be to create a business rule that runs which updates the RITM with the task information.