Hey chief, would you please check if the @ user name works in the Arena forums (especially contest submissions) and if it does, will you check if the issues of not getting a notification when @ at is because of multiple @s in a row in the same post?
Another thing to check: if users who restrict some information about themselves that use the @ command might also be a cause of not getting a notification.
Another thing to check: if users who restrict some information about themselves that use the @ command might also be a cause of not getting a notification.