How to bind an infoblock element to another section when it is changed?

Question

1C-Bitrix

Hello.

When I change an element, I'm trying to make it automatically bind in addition to one more section.
I can't figure out what's wrong, please tell me:

AddEventHandler("iblock", "OnAfterIBlockElementUpdate", Array("AddToSection", "OnAfterIBlockElementUpdateHandler"));

class AddToSection 
{
  public static $disableHandler = false;
  
  function OnAfterIBlockElementUpdateHandler(&$arFields)
  {
    if (self::$disableHandler)
      return;

    CModule::IncludeModule("iblock");

    if( $arFields["ID"] > 0 && $arFields["IBLOCK_ID"] == "21")
    {
      $el = new CIBlockElement;

      $arSections = array();

      if(in_array("104", $arFields["IBLOCK_SECTION"], true))
      {
        file_put_contents("/home/bitrix/www/bitrix/php_interface/log.txt", "Товар уже находится в папке 104");
      }
      else 
      {
        $arSections = array_push($arFields["IBLOCK_SECTION"], "104");

        $arLoadProductArray = Array(
          "IBLOCK_SECTION" => $arSections,
        );

        self::$disableHandler = true;

        $res = $el->Update($arFields["ID"], $arLoadProductArray);
      }
    }
  }
}

Binding to any section generally disappears in this case. And for some reason, $arSections displays not an array of sections, but the number 2...