Do you have any JavaScript that is modifying the form? If so, we’ll need to see that.
Any time you change the number of forms in a form set, you need to update the management form. See https://docs.djangoproject.com/en/3.0/topics/forms/formsets/#understanding-the-managementform
If you are adding new forms via JavaScript, you should increment the count fields in this form as well.
Also, if you included the empty form in the <form>, that might also do it. The empty form should be in a completely separate section.