In a ticket (RE/IN/PR) that has a classic workflow task, the workflow task is updated (example: new description). This information appears in the activity log of the ticket. However, the last modified date (last_mod_dt) is not updated in the summary information section.
This is by design.
Release : 17.1
Component : SERVICE DESK MANAGER
In the scenario described in the case, the main object/resource is the ticket. This object has relationships with other objects (SREL attributes) for example status, priority, wf tasks, etc. When any of these relations change, such as adding or removing them, then the main object is updated. But when the secondary objects details is updated, it doesn't have an effect on the main object. It is independent. Thus, in this scenario, updating the workflow tasks description does not update the ticket last mod date. Thus working as designed.