It is expected behavior of this feature in the new version of Meta Box. You can add a new term even do not select any terms, the new term will be assigned to the post (object) after publishing/updating the post.
Thank you, Peter! But it seems you misunderstood me.
This happens when I create a new term, not when I add a term to the post. The custom taxonomy has a required field, so I'm expecting to get a warning before creating the new term that stops the creation procedure. But what happens is the term is being created even when the required field is blank.
Here's how to reproduce the issue:
Try to create a new custom taxonomy MY_TAXONOMY, then create some fields for it and make one of them required. Then visit this page: