I don't know whether this is an actual bug or it's just happening to me, but it never happened before now, as far as I can remember.
What happens is that someone pings me in chat, I read the message because I'm already in the chat room, and the conversation goes on. After a while, I get the red ball notification in the MultiCollider in the Main site (which one is not important for this), only to discover it's a relatively old message that I've already seen and replied to minutes earlier.
What is happening?
Adding some detail from the comments: I am using a computer, not on mobile. When I see the notification, I assume it's about some new event, but the preview makes me realize it's an old chat message.