Same thing happening for me. I thought maybe it was my browser settings.It's been doing this for a while. Open/read alerts (say there are three of them), then the alert number disappears. Go to another part of the site, the 'three' returns for a second or two, then goes away.
I'm not sure there is a simple way to stop it from happening without having to 'pop the hood and tinker around' (pop the bonnet if you prefer).