[Plugin: Related Posts Picker] Changes doesn't apply

Hello.
I’m using related-post-picker plugin with custom post type. I just added this:
add_meta_box( 'rel-post-div', 'Related Posts', 'rpp_inner_custom_box', 'wpsc-product' );
after:
add_meta_box( 'rel-post-div', 'Related Posts', 'rpp_inner_custom_box', 'post' );

So meta box is now displaying at the custom post type edit page, and plugin works if i tick related posts first time and then update/publish the post. But when i try change related posts (tick new or untick) it doesn’t aplly at all! What i’m doing wrong?

https://www.ads-software.com/extend/plugins/related-post-picker/