Hey folks, sorry for the belated reply. Checked with engineering and found out the following conditions change the modified_at timestamp:
- story is created on a task
- story is trashed on a task
- attachment is trashed on a task
- task is assigned or unassigned
- custom field value is changed
- the task itself is trashed
- Or if of the following fields are updated:
- completed
- name
- due_date
- description
- attachments
- items
- schedule_status
The task moving to a new container (project, portfolio, etc) will not affect its modified_at timestamp.