In contextual search, the field to which knowledge article can be attached can be configured for different tables, but this functionality doesn't work when using knowledge/book icon-search. 

1. Open the Table Configuration record for
Table: Incident [incident]
UI Type: Platform
https://<instance-name>.service-now.com/nav_to.do?uri=cxs_table_config.do?sys_id=b3ca5632c32121005655107698ba8fba

From its related list, open the Search Action Configuration record 'Attach' for 'KnowledgeSearchResource'
https://<instance-name>.service-now.com/nav_to.do?uri=cxs_ui_action_config.do?sys_id=63c5b77587a013005d90bba826cb0bc8

Confirm that:
'Use custom field for attach note' = true
'Attach note field' = work_notes

2. On an incident form, Right-click the header > Configure > Form Layout and add the 'Contextual Search Results' macro

3. Open an existing incident, click on the 'Related Search Results' button and then on 'Attach' next to one of the articles

>> The link to the KB article will be added to the work_notes, as expected.

4. On the incident form, Right-click the header > Configure > Form Layout and remove the 'Contextual Search Results' macro

5. Open an existing incident, click on the 'Search Knowledge' icon next to the short_description, select a KB article in the popup and then click on the 'Attach to Incident' button

>> The link to the KB article will be added to the comments, which is incorrect

No workaround is available for this issue. The recommendation is to use contextual search where the field to which article can be attached can be configured for different tables.

(Documentation link: https://docs.servicenow.com/bundle/madrid-platform-administration/page/administer/contextual-search/task/t_AttachAnArticle.html)