(This is surely a bug.)
When I refresh the page after doing so, "mark all as visited" appears grayed out and is not clickable (for a while--I don't know how long, but after enough time/refreshing/articles unvisited, it becomes clickable again).
It's not a script or gadget. All of mine are turned off (due to troubleshooting related to WE 2017).
Firefox 50.1.0 on Windows 10.