Problem when putting space on the name of the anchor

For people encountering this problem: This can also be fixed without the patch and instead add a custom text format filter in your custom module.

my_module/src/Plugin/Filter/AnchorFilter.php

namespace Drupal\my_module\Plugin\Filter;

use Drupal\Component\Utility\Html;
use Drupal\filter\FilterProcessResult;
use Drupal\filter\Plugin\FilterBase;

/**
 * Custom text format filter to fix anchor id's with spaces.
 *
 * This will:
 * - Replace spaces in anchor link ID's by hyphens (-).
 * - Add unique postfix when link IDs are duplicated on a page.
 *
 * @Filter(
 *   id = "filter_anchor",
 *   title = @Translation("Anchor Filter"),
 *   description = @Translation("Help this text format fix id's with spaces between"),
 *   type = Drupal\filter\Plugin\FilterInterface::TYPE_TRANSFORM_IRREVERSIBLE,
 * )
 */
final class AnchorFilter extends FilterBase {

  /**
   * Creates unique ids for each anchor link.
   */
  public function process($text, $langcode): FilterProcessResult {
    $dom = Html::load($text);

    $links = $dom->getElementsByTagName('a');
    /** @var \DOMElement $link */
    foreach ($links as $link) {
      $this->cleanLinkId($link);
    }

    return new FilterProcessResult(Html::serialize($dom));
  }

  /**
   * Cleanup given link DOM Element.
   *
   * This will clean the ID only when:
   * - Link is not a normal link (with href).
   * - Link has ID.
   * - Unique ID will be generated from link ID.
   *
   * @param \DOMElement $link
   *   The link element.
   */
  private function cleanLinkId(\DOMElement $link): void {
    if (!empty($link->getAttribute('href'))) {
      return;
    }

    $linkId = $link->getAttribute('id');
    if (!$linkId) {
      return;
    }

    $link->setAttribute(
      'id',
      Html::getUniqueId($linkId)
    );
  }

}

Enable this filter in your respective text formats.