HR Knowledge users cannot edit other KB articles falling under HR knowledge base.
The expected behavior is roles specified in "Can contribute" under the knowledge base should work. Any user having that role should be able to edit knowledge articles falling in the same KB and state of KA is in Draft.
This is the expected behavior.
When you mention can contribute criteria on the knowledge base please see that the user is able to create new articles. Now you are not able to edit an already created article which is in the draft state because of an OOB ACL.
If you publish an article by impersonating the author and then impersonate back to User notice you will be able to check out and hence be able to contribute to the article.