Got a response from Ninja Support which is of immense help.
Thank you for reaching out to us. The _1 is being appended because the form was added through the Append a Ninja Form met-box. It is a feature rather than a bug, so to speak :blush: . If this is causing a validation problem or other form issues, please let us know. Otherwise, the form is working as expected. To remove the _1 add the form via shortcode instead of with the meta-box.