@paul-thornton I seem to get the notification now, but I'm guessing it's the mentions specifically, since I turned on the "replies in topics you watch".
I'm pretty sure it's not browser related since it's not in the notifications view, but I'm using google chrome Version 71.0.3578.98
I'm guessing you get the notifications because of the reply and you're watching the topic, not because of the mention. I'm going to put mentions on emails only and replies on notifications. That should send me an email when I get mentioned and a notification because there's a reply if you reply to this topic.