I often am in chat to be reachable by users and other moderators.
Sometimes I happen to receive a ping while I am online.
For the past few months it often happens that I cannot dismiss the notification (i.e. the "1" on the top right corner of my avatar located on the bottom left corner of the screen) by clicking on it as usual, but I first have to reload the page.
The only alternative is to post a reply that will automatically dismiss the notification, but I don't always have a reply to give (or is not needed).
I have tried to observe if I could identify any specific pattern, but I cannot see any relationship with how long the chat tab has been open, or how many chat rooms I currently am in, or anything else.
I have some browser extensions (using the latest Firefox b.t.w.), but NoScript is not active while navigating chat, and in general I have not added any extensions to my usual set (ABP, Ghostery, Privacy Badger, and Containers) recently.