URL prefix breaks reverse proxy

We have an installation of h5p that sits behind a reverse proxy.

This code on line 1998 of h5p.classes.php causes many things such as the video controls in the video player to break:

  /**
   * Combines path with cache buster / version.
   *
   * @param array $assets
   * @return array
   */
  public function getAssetsUrls($assets) {
    $urls = array();

    foreach ($assets as $asset) {
      $url = $asset->path;

      // Add URL prefix if not external
      if (strpos($asset->path, '://') === FALSE) {
        $url = $this->url . $url;
      }

      // Add version/cache buster if set
      if (isset($asset->version)) {
        $url .= $asset->version;
      }

      $urls[] = $url;
    }

    return $urls;
  }

This line is the problem:

$url = $this->url . $url;

This results in a hard-coded URL that doesn't work when behind a reverse proxy.

Is there a reason why a fully-qualified hard-coded URL is needed here?

Is there any way to change this to a relative path?