Visual Revisions fail to restore meta controlled by Classic Meta Boxes #78130

<?php
/**
 * Plugin Name: MyGuten Separate Meta Test
 * Description: Classic meta box + Gutenberg sidebar meta field, both REST and revision enabled.
 * Version: 1.0.0
 */

if (!defined('ABSPATH')) {
    exit;
}

const MYGUTEN_CLASSIC_META_KEY = 'myguten_classic_meta_field';
const MYGUTEN_GUTENBERG_META_KEY = 'myguten_gutenberg_meta_field';

/**
 * Register meta fields.
 */
add_action('init', function() {
    $common_args = [
        'type'              => 'string',
        'single'            => true,
        'show_in_rest'      => true,
        'revisions_enabled' => true,
        'sanitize_callback' => 'sanitize_text_field',
        'auth_callback'     => function () {
            return current_user_can('edit_posts');
        },
    ];

    register_post_meta('post', MYGUTEN_CLASSIC_META_KEY, $common_args);
    register_post_meta('post', MYGUTEN_GUTENBERG_META_KEY, $common_args);
});

/**
 * Show meta fields in revisions
 */
add_filter( '_wp_post_revision_fields', function( $fields ) {
    $fields[ MYGUTEN_CLASSIC_META_KEY ] = 'Classic Meta Field';
    $fields[ MYGUTEN_GUTENBERG_META_KEY ] = 'Gutenberg Meta Field';
    return $fields;
} );

/**
 * Enqueue Block Editor Assets
 */
add_action('enqueue_block_editor_assets', function() {
    $script_path = plugin_dir_path(__FILE__) . 'sidebar.js';
    wp_enqueue_script(
        'myguten-meta-sidebar',
        plugin_dir_url(__FILE__) . 'sidebar.js',
        ['wp-plugins', 'wp-edit-post', 'wp-element', 'wp-components', 'wp-data', 'wp-core-data'],
        file_exists($script_path) ? filemtime($script_path) : '1.0.0',
        true
    );
});

/**
 * Classic Meta Box Setup
 */
add_action('add_meta_boxes', function() {
    add_meta_box(
        'myguten_classic_meta_box',
        'Classic Meta Field',
        'myguten_render_classic_meta_box',
        'post',
        'side'
    );
});

function myguten_render_classic_meta_box($post) {
    $value = get_post_meta($post->ID, MYGUTEN_CLASSIC_META_KEY, true);
    wp_nonce_field('myguten_save_classic_meta', 'myguten_nonce');
    
    printf(
        '<input type="text" name="%1$s" value="%2$s" style="width:100%%;" />',
        esc_attr(MYGUTEN_CLASSIC_META_KEY),
        esc_attr($value)
    );
}

/**
 * Save Classic Meta
 */
add_action('save_post_post', function($post_id) {
    if (!isset($_POST['myguten_nonce']) || !wp_verify_nonce($_POST['myguten_nonce'], 'myguten_save_classic_meta')) {
        return;
    }

    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) return;
    if (!current_user_can('edit_post', $post_id)) return;

    if (isset($_POST['myguten_classic_meta_field'])) {
        update_post_meta($post_id, MYGUTEN_CLASSIC_META_KEY, sanitize_text_field(wp_unslash($_POST['myguten_classic_meta_field'])));
    }
});