When there is no 'New' button (action-assignment) configured for the related-list table but by-default the 'New' button is showing in that related-list, therefore it is pointing to the 'New' button from the 'Global' table, as Global table's action-buttons will appear on all the forms/related-lists unless any specific ones are configured for the target table.

Orlando onwards

1. From the application-navigator, navigate to -- Workspace Experience > Actions & Components > Related List Actions
2. Apply this filter:
   - Action label | is | New
   - Table | is | Global
3. Open that 'sys_declarative_action_assignment' record
   
4. If you are not in 'Global' application, you will not notice any 'New' button in the 'Action Exclusions' related-list
5. Switch to the Application = Global, if not already > now the 'New' button will appear
6. Click on 'New' > notice the "Action assignment = create-new" will be auto-populated
7. Now if you try to find the target table that is in different Application, example: a table under Customer-Service application, it will not be available in the list of tables
8. Therefore again switch to the required application, example = Customer Service, immediately you will find that table in the list
9. Set the below values >>
   - Table = related_list_table_name
   - Exclude this table = checked
   - Exclude all child-tables = optional (based on your requirement)
10. Save the record
    
    Now should notice the 'New' button (action) is unavailable in the related-list.

You can also check this article for reference: https://hi.service-now.com/kb_view.do?sysparm_article=KB0827814