SLA Actual elapsed time and Actual elapsed percentage are at '0' even when the task_sla Stage is "In progress" - why?

The user has an incident record and a task_sla on that incident record. The task_sla record, in both the related list and task_sla record itself, is showing that the Actual elapsed time and Actual elapsed percentage are at '0' even when the task_sla Stage is "In progress".

System property "glide.sla.calculate_on_display" is set to a value of "false".

As mentioned above, it was found that by setting the above system property to a value of "true", the behavior no longer occurs.

This is because setting the system property to a value of "true" ensures that the timings in the Task SLAs are updated each time the task form is viewed.

Otherwise, the user has to manually go into the task_sla and click the "Refresh" UI Action to see updated timings (e.g. when the system property is set to "false").