Found a workaround below. I guess it has to do with a bug in the WordPress core. This will list ALL users in posts, so be cautious.
// Filter to fix the Post Author Dropdown
function author_override( $output ) {
global $post, $user_ID;
// return if this isn't the theme author override dropdown
if (!preg_match('/post_author_override/', $output)) return $output;
// return if we've already replaced the list (end recursion)
if (preg_match ('/post_author_override_replaced/', $output)) return $output;
// replacement call to wp_dropdown_users
$output = wp_dropdown_users(array(
'echo' => 0,
'name' => 'post_author_override_replaced',
'selected' => empty($post->ID) ? $user_ID : $post->post_author,
'include_selected' => true
));
// put the original name back
$output = preg_replace('/post_author_override_replaced/', 'post_author_override', $output);
return $output;
}
add_filter('wp_dropdown_users', 'author_override');
credit goes to here