You may have noticed that, when setting the temporal membership for an existing user of a group via the WI, the approval request task submitted to the Approver indicates immediate removal. Instead of incicating the schedule for removal, the approved task revokes the group membership instantly.
This is a product defect (TF00200308). The WI approval request task is stripping the ScheduledOperation-SetTime control.
Set the temporal group membership, when approval is required, via the AR MMC.
This will be fixed on a future release of the product. Please refer to this article for updates or contact support referencing the defect ID TF00200308.