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:
example.com/wp-admin/edit-tags.php?taxonomy=MY_TAXONOMY
Now try to add a new term without filling in the required field; the term will be added directly without warnings or stopping the creation procedure.
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