WordPress query parameter [author_in] not working

When using the WordPress query parameter “author__in” together with your plugin, WordPress only returns posts where the given author is mentioned first in the list of multiple authors.

That is, if a post has “Alfred” and “Ben” set as authors (in that order), a query with author_in set to “Ben” will not return that post, but will return it if author__in is set to “Alfred”.
If the order of the authors are swapped, so it is “Ben” and “Afred”, the opposite will be the case, queries with author__in set to “Ben” will return the post, and author_in set to “Alfred” will not.

This is a problem when using third party plugins like FacetWP, since it relies on making queries with author__in, and thus will return results based on the order the authors has been added to the posts (mistakenly ignoring those where the author in query is not set as the first author).

Is this a bug that can be fixed, or a feature that there is some kind of work around for? Or is your plugin just not compatible with third party plugins like FacetWP?

Thank you.