Recently restarted my premium subscription and am seeing some stories showing up in the feed as brand new posts with a bookmark already set.
Review the bookmarks page in my library and see several of them with a yellow 0/## showing up in the list.
My suspicion is the bookmark tables aren't getting cleaned up when a title has been removed or before the story id that the bookmark table is probably keyed to are reassigned to a new title.
I've left the bookmarks alone for now so you'll be able to see them in the current state.