The Stop time field is not populated on the form of a completed task SLA
The name of the schedule on the SLA definition was changed.
Existing response task SLAs created prior to the name change had picked up the old schedule name.
Schedule is factored in to the calculation of the Stop time value.
When calculating the Stop time value it couldn't find the schedule because the schedule name had been changed and therefore no date value was entered in the Stop time field
Running repair SLA on the affected records fixes the issue.