There seems to be a fundamental difference between get_user_by() and get_users(). Where the former creates a new WP_User object and initiates it via an array of data, the latter puts the actual database result into the WP_User's $data property via its constructor.

The end result is that the set of available properties is different. As an example, when a user is fetched via get_users(), it knows about its nickname. If it is fetched by get_user_by(), the nickname property does not exist.

This PR changes the UserFetcher to rely on get_users() (instead of get_user_by()), to make all properties available.