Widgets moved by calling the sidebars endpoint

[davit16]

The GET /wp/v2/sidebars endpoint moves the widgets permanently, if for some reasons, one or more sidebars are not registered for WP REST. Moreover, it does it even if it returns a 401 status code.

To reproduce the issue:

1. Have Gutenberg disable with the "Disable Gutenberg" plugin

2. Add this code to theme's functions.php

   function test_register_sidebars() {
           if ( strpos( $_SERVER['REQUEST_URI'], '/wp-json/' ) === false ) {
                   register_sidebar( [
                           'name'          => 'Sidebar 1 - not registered on WP REST',
                           'id'            => 'sidebar-1',
                           'before_widget' => '<section id="%1$s" class="widget %2$s">',
                           'after_widget'  => '</section>',
                           'before_title'  => '<h2 class="widget-title">',
                           'after_title'   => '</h2>',
                   ] );
           }
   
           register_sidebar( [
                           'name'          => 'Sidebar 2',
                           'id'            => 'sidebar-2',
                           'before_widget' => '<section id="%1$s" class="widget %2$s">',
                           'after_widget'  => '</section>',
                           'before_title'  => '<h2 class="widget-title">',
                           'after_title'   => '</h2>',
           ] );
   }
   add_action( 'widgets_init', 'test_register_sidebars' );
   

3. Add widgets to "Sidebar 1"

4. Visit GET /wp-json/wp/v2/sidebars