The way Xenforo works is that an @ triggers an alert, however, an [USER][/USER] tag negates the alert, and it has priority over the @ tag. If you try to reply to any post with a mention in it, you'll see that it automatically comes with the [USER][/USER] tags in it, that's to avoid you mentioning someone over and over again when replying to a given post. However, when you copy paste, the [USER][/USER] tags don't get carried over (but the @ does!), so they should trigger the alert normally~
Ohhh! Here i am, learning something new about this site in a thread like this~ Haha, thanks for that info :'p
Well, that's just what I learned by experimenting with things! xD Xenforo made a pretty good system with the alerts~
I'm not sure tbh, I remember making multiple threads to try out different things, but I'm not sure on the when I did them, maybe I already knew most stuff by then! xD I tested a lot in old AG though~