This has been mentioned before and is working as intended. Much of the world doesn't use two ALT keys. Instead, they have one ALT key (left) and one ALT GR key (right). mIRC was updated in one of the recent versions to support ALT GR. That removed some of the control that people are used to who do use two ALT keys. It's been requested that mIRC determine which keyboard layout you're using and allow two ALT keys if you're using US layout or any other layout that has two ALT keys. From what Khaled said in the past about it, I don't think this is a high priority for him, though.