Hello James,
Is that plugin is working on the featured image but not working on Related Post? Related post image will appear from the featured image. If you add the multiple featured image, the related post will not aware that which featured image need to appear there. So, you need to customize the related post code.
Find the code inside acmethemes/hooks/related-post.php and try to customize the theme. We haven’t tested all the plugin of the WordPress repo, so it may arise conflicts on the plugin.
Thank You