Let’s say you have a private conversation with a member and the appropriate topic /t/[topic-name]/[numbers] is open. Now the member will send you a new message in this topic. The green indicator for new private messages (on the Avatar) goes to (1). You directly see the message in the topic, since it is already open. Now you switch to another topic and the green (1) is still there.
I’m experiencing this all the time. I’ll making a recording next time I receive a PM (feel free to send me a test PM; too lazy to do that myself with a test user during the weekend!)
In short, the notification is still there when I open a message, when really it should be cleared as soon as you’ve opened the message.
I don’t have a consistent way to repro it, but it happens quite often to me. There’s definitely more to it then requiring to actually read the post, when it happens to me scrolling through the post doesn’t help.