【devise】registrationコントローラのcreateアクションでsuperが使えない

Question

#概要
gem 'devise' を使ってユーザー新規登録の実装をしています。
devise用のテーブルにuser_nameカラムを追加してあります。
表題の通り、以下の実装で registration#create で UnknownFormat エラーを吐かれてしまいます。
(Devise::RegistrationsController の createアクションを使いたい)
理由は何故でしょうか。
#質問の背景
以下の registration#create の実装で期待通りの挙動ができたのですが、
使い方が芳しくないと指摘を受けたため。

def create
   @user = User.new(registration_params)
   if @user.save
      redirect_to new_user_session_path, success:'登録完了'
   else
      render :new
   end
end
def registration_params
   params.require(:user).permit(:user_name, :email, :password, :password_confirmation)
end

###application_controller.rb

class ApplicationController < ActionController::Base
 before_action :configure_permitted_parameters, if: :devise_controller?
 protected
 def configure_permitted_parameters
   devise_parameter_sanitizer.permit(:sign_up, keys: [:user_name])
 end
end

###registrations_controller.rb

class Users::RegistrationsController < Devise::RegistrationsController
 before_action :configure_sign_up_params, only: [:create]
 
 def new
   super
   @user = User.new
 end
 def create
#ActionController::UnknownFormat in Users::RegistrationsController#create
   super
 end
 protected
 def registration_params
   params.require(:user).permit(:email, :password, :password_confirmation)
 end
 def configure_sign_up_params
   devise_parameter_sanitizer.permit(:sign_up, keys: [:attribute])
 end

###config/routes.rb

Rails.application.routes.draw do
 # For details on the DSL available within this file, see http://guides.rubyonrails.org/routing.html
 devise_for :users, skip: :all
 devise_scope :user do
   root to: "users/sessions#new", as: :new_user_session
   get "sign_up", :to => "users/registrations#new", as: :new_user_registration
   post "sign_up", :to => "users/registrations#create", as: :user_registration
   get "sign_out", :to => "users/sessions#destroy", as: :destroy_user_session
   get "password/new", :to => "users/password#new", as: :new_user_password
 end
end

##app/views/users/registrations/new.html.erb

h2>新規登録</h2>
<%= form_for(resource, as: resource_name, url: new_user_registration_path(resource_name)) do |f| %>
 <%= render "devise/shared/error_messages", resource: resource %>
 <div class="field">
   <%= f.label :user_name %><br />
   <%= f.text_field :user_name, autofocus: true, autocomplete: "name" %>
 </div>
 <div class="field">
   <%= f.label :email %><br />
   <%= f.email_field :email, autofocus: true, autocomplete: "email" %>
 </div>
 <div class="field">
   <%= f.label :password %>
   <% if @minimum_password_length %>
   <em>(<%= @minimum_password_length %> characters minimum)</em>
   <% end %><br />
   <%= f.password_field :password, autocomplete: "new-password" %>
 </div>
 <div class="field">
   <%= f.label :password_confirmation %><br />
   <%= f.password_field :password_confirmation, autocomplete: "new-password" %>
 </div>
 <div class="actions">
   <%= f.submit "新規登録" %>
 </div>
<% end %>
<%= render "users/shared/links" %>