it's a problem from the original Co Authors plugin. It's surely a bug.
In my theme I use the_author_posts(); to get author's posts count, but with co-authors plus installed it seems to get co-authors posts count of that author. For example I published 200 posts without the plugin and it showed 200 posts, then I activate the plugin and it shows 2-3 posts, that are published posts with the plugin activated.
So could you solve this problem?