Why doesn't the Globals condition work?

Question

1C-Bitrix

I registered the following in init.php, there is a checkbox in the property with the value Y, for some reason it does not change the content.

AddEventHandler("main", "OnEndBufferContent", "ChangeMyContent");

function ChangeMyContent(&$content)
{
global $APPLICATION;
if ($GLOBALS["NO_INDEX"] == "Y"){ $content = str_replace('noindex, follow','index, follow',$content); }

Should replace the content when the checkbox is enabled.