After a whole lot of trial and error, I think I’ve found that the only way to send a notification to an @ mentioned user in a timed topic posting, is if they’re actively “Watching” the category into which the topic is published. Even setting the category to “Tracking” wouldn’t send the notification, which does seem like a bug.
A simple workaround could be to create and select a new category in the “default categories watching” user preference. Then any topics you publish into that category would send a notification to any user mentioned.