The behavior where Attach is not Available for unsaved records is by design.
Knowledge articles are only attached to saved/existing incidents. Since the incident may not be saved/submitted, not having Attach button makes sense.
Before the "Calgary" version, users were able to attach a KB article to an unsaved record.
The unsaved record would not have a proper sys_id before being saved and therefore when the record is abandoned (not saved)/or saved afterward an m2m malformed record would exist.
This is a general platform issue when adding related records to an unsaved record therefore it's better to not allow that.
As a result of this change, we would not allow users to attach a KB article to an incident that has not yet been saved.