In testing, what happens is if you assign a writer to a post, the writer then has to accept the post in order for you to add that writers name as an "Author" where Co-Authors Plus takes over the Author drop down.
If I were to disable Co-Authors Plus, maybe the assignment would work and we could publish with just that one author's name being switched.
But, Co-Authors Plus is needed, because we have multiple journalists on a story.
I think the assumption is that without co-authors plus, yes, you could probably change the author name and that would save correctly without the reporter having to accept. But, with co-authors plus, there is something between all three plug-ins that's preventing that from happening.
If you have all three installed...try then to change the Author's name in the widget under the post. It uses live type feature. You enter the new authors name and hit save. But it then reverts back to the original owner, because the new author hasn't yet accpted in their dashboard.