Mandatory checks failed for Multi-selection field in UI

I have created a custom post type and added a Relationship field with custom simple list. I set this field to be mandatory with multi-selection and presented in check boxes.

However, I found the mandatory checks is not preformed when I save drafts or publish the post. But for other relationship field, seems it does not have this problem. How I can solve this problem? Seems to be a bug for the plugin.