theme.json is altering Group block HTML

I was able to prevent the unwanted altering of HTML by using the render_block_core/group filter, and a copy of wp_restore_group_inner_container, without the JSON support check.

public static function my_add_block_group_inner( $block_content, $block ) {
        $tag_name                         = isset( $block['attrs']['tagName'] ) ? $block['attrs']['tagName'] : 'div';
        $group_with_inner_container_regex = sprintf(
            '/(^\s*<%1$s\b[^>]*wp-block-group(\s|")[^>]*>)(\s*<div\b[^>]*wp-block-group__inner-container(\s|")[^>]*>)((.|\S|\s)*)/U',
            preg_quote( $tag_name, "https://wordpress.stackexchange.com/" )
        );

        $replace_regex   = sprintf(
            '/(^\s*<%1$s\b[^>]*wp-block-group[^>]*>)(.*)(<\/%1$s>\s*$)/ms',
            preg_quote( $tag_name, "https://wordpress.stackexchange.com/" )
        );
        $updated_content = preg_replace_callback(
            $replace_regex,
            static 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_core/group', 'my_add_block_group_inner' );