I've decided to finally clean up my notifications after noticing that some of the older ones (older than two months or so) were duplicated. After reloading the page a few times (the duplicated messages didn't have the "dismiss" button when appended to the page using the "more" button), I found a lot of the notifications missing without me doing anything to dismiss them. They don't reappear after I refresh the page, either. I'm not worried about them gone, but I'd like the future notifications not to do so.
Happened to me a few month ago as well.