Post Orlando, users are unable to attach the document to the KB article , when the status of the article is set to published, retried, out_dated .
Client script onload "Hide Attachment Link when read only" introduced in Orlando which is limiting the attach button
This is the expected behavior. When an article is in Published state, the form is read-only until the article is checked out. Then a new version is created and you can add attachments to the new version.
That is the intended behavior with knowledge versioning enabled, if you wish to add attachments or make edits then they can disable versioning feature.
We wouldn't recommend disabling the client script, because it would be customization and might lead to upgrade issues in the future.