This is by design.
Release : 17.2
Component : SERVICE DESK MANAGER
The activity log can not be created for the Workflow tasks which are updated automatically (i.e through an action macro). Like GRPSTART/GRPEND tasks gets updated automatically. Similarly if an Approval task is updated, its next task gets updated to PENDING status.
So for these kind of updates, out of the box, there will not be any log created.