Unable to rebuild a form in submit handler
What exactly do you expect to happen when it's "rebuilding"?

I should simply call buildForm() again, but that will build the same form, so visually, you won't see a difference. The only ondication would be that it's doing only a POST request, while a non-rebuilding form submission does a redirect to the same page after submitting.

If you e.g. want to see additional rows in your list, you need to store the desired max with $form_state-&gt;set('max_rows, 5) and then in your buildForm() method, read it again from there with get()