Author archive page shows first author only as title

I’ve implemented this plugin, and it’s working well, with one exception. When I go to an author archive page, if the author is a co-author and the other author is listed first on the first post, that author’s name shows as the title. For example, if I’m looking at Bob Lee’s archive page and his first post is one written by Jane Andrews and Bob Lee, the title of Bob Lee’s archive page will be Jane Andrews. Has anybody run into this issue?

Here is the code that creates the title (from Twentyfifteen, which I’m customizing):

elseif ( is_author() ) {
$author_id = intval( get_query_var(‘author’) );

$title = sprintf( $args[‘authortitle’], get_bloginfo(‘name’), $args[‘separator’], get_the_author_meta( ‘display_name’, $author_id ) );
$href = get_author_feed_link( $author_id );