Mastodon Behavior, Subtooting
Further testing reveals in the web UI that if I reload the page, the notification reappears. Clicking on it will again make it disappear, but then reloading the whole page will restore it. I'm guessing that this means there's an order-of-operations problem in the loading of notifications.
Interestingly, right-clicking on the time of the notification takes me to a new window which shows all of the posts. This may be a federation issue. @Gargron?