This has popped out for quite long time now. I didn’t bothered report it because I was sure it is an issue of DiscourseHub and they know there is problems with that too.
It will disapper at some point when enough time has passed. Once helped when I deleted all messages sent by system.
But if it bothers browsers too then it have to come from Discourse itself.
I’ve noticed the same thing. It seems like it corresponds to when a “New advice on your site dashboard” message would have been sent, but the last one of those that actually shows up in my inbox is from August 2021.