What do you mean by a topic is opened? When someone starts a new topic or when you visit (open) a topic you haven’t read yet?
What’s the “new” category? Are you talking about the new tab?
There’s only one way we don’t show the notification of a mention, that is when the other person has muted the user or topic. Otherwise it should always show the notification.
Again, it depends on the notification level of said topic.
That’s my point. Even though I sometimes overlook that it’s actually a new message or reply I did actually see it. Otherwise, I have not seen this issue directly.
That being said, I will make note next time I see something strange.