Form Ajax submission: erratic behavior
What you can see in the Gif looks more like the validation of the form in javascript not in ajax.

Normally you decide in the submit button by including the option 'wrapper' =&gt; 'ajax-wrapper-id', if you want to use the wrapper method and return in the ajax callback the new version of the form or parts of it, or use the response method and build an ajax response in the callback. Doubt that you can mix it like this.

If you use the wrapper method you don't need to rebuild the form, because it is already rebuilt, when it reaches the ajax callback.