This is a very small thing and likely it's me: since the past few months, I've had to tap the "mark as read" twice to get it to do that. The first time, it goes away, then quickly reappears. The second time it's gone. This occurs on mobile or laptop/desktop and doesn't seem to matter with the browser. The gif shows it (Firefox, laptop), though it's hard to see because it is cycling around as gifs do.