WordPress/wp-includes/block-supports/layout.php

<?php
/**
 * Layout block support flag.
 *
 * @package WordPress
 * @since 5.8.0
 */

/**
 * For themes without theme.json file, make sure
 * to restore the inner div for the group block
 * to avoid breaking styles relying on that div.
 *
 * @since 5.8.0
 * @access private
 *
 * @param string $block_content Rendered block content.
 * @param array  $block         Block object.
 *
 * @return string Filtered block content.
 */
function wp_restore_group_inner_container( $block_content, $block ) {
	$group_with_inner_container_regex = '/(^\s*<div\b[^>]*wp-block-group(\s|")[^>]*>)(\s*<div\b[^>]*wp-block-group__inner-container(\s|")[^>]*>)((.|\S|\s)*)/';

	// TODO: Add check for theme.json presence.
	if (
		'core/group' !== $block['blockName'] ||
		1 === preg_match( $group_with_inner_container_regex, $block_content )
	) {
		return $block_content;
	}

	$replace_regex   = '/(^\s*<div\b[^>]*wp-block-group[^>]*>)(.*)(<\/div>\s*$)/ms';
	$updated_content = preg_replace_callback(
		$replace_regex,
		function( $matches ) {
			return $matches[1] . '<div class="wp-block-group__inner-container">' . $matches[2] . '</div>' . $matches[3];
		},
		$block_content
	);
	return $updated_content;
}

add_filter( 'render_block', 'wp_restore_group_inner_container', 10, 2 );