Set Unpublish, Publish is set automatically on save

Running Drupal 6.26 on a site I just inherited. Server is Debian, PHP v5.3. Site also running Domain Access.

Have installed this module with the goal of allowing subsite users to Unpublish own content without giving them Publish (there is an approval workflow where a moderator publishes approved content).

When I enable an Unpublish permission (in this case it is Unpublish own (nodetype) and save, Publish is automatically selected for that node type as well.
Conversely, if I enable Publish and save, Unpublish is *not* automatically selected.

My understanding of this module was that I should be able to assign the two permissions individually to separate roles. Is the behaviour I'm seeing expected, or is this a bug?

Thanks.