The Search Action Configuration of both Incident and Case(sn_customerservice_case) table for 'Attach' is set to 'work_notes'.

When a user attaches a KB with an incident through Agent Workspace, the attachment link appears under work notes which is expected behavior.
But, when a user attaches a KB with a Case(sn_customerservice_case) through Agent Workspace, the attachment link appears under comments instead of work notes which is incorrect behavior.

1) login to OOB NY instance
2) set the Search Action Configuration of both Incident and Case(sn_customerservice_case) table for 'Attach' is set to 'work_notes'.
3) go to agent workspace
4) using list go to open incidents and open any record
5) go to agent assist >> attach any KB article - Observe the work note field is populated with the attachment link.
6) within Agent Workspace, using list go to open Case(sn_customerservice_case) and open any record
7) go to agent assist >> attach any KB article - Observe the comment field is populated with the attachment link, not the work_notes.

To resolve the reported issue, go to the Search Action Configurations (cxs_ui_action_config) table.
Filter out the records where Base configuration = Case [sn_customerservice_case], action label = Attach

Set the 'Use custom field for attach note' field to true and set the 'Attach note field' to 'work_notes'.