I dont know when it changed of if it have ever been like that, but when you edit a media file, then the meta boxes are there from the plugin, but it is not saved because you are using

add_action( 'wp_insert_post', array( $this, 'save_postdata' ) );
in file class-metabox.php

So if you just add this below
add_action( 'edit_attachment', array( $this, 'save_postdata' ) );

It all works again.