I really don't know what it is, but in my Hufflepuff club on NL I have a game topic, and every time someone replies to that topic, all previous notifications are wiped. It always happens like this. I could have 3 unread and 2 read normal topic and article notifications, but when someone replies to this game topic and I refresh the page, all those notifications are gone.
Now, I know that sometimes the notifications list randomly wipes, but not this often, and certainly not every time someone replies to a topic.
Customer support service by UserEcho