I can’t replicate this, but it happened only with snoozed tabs.
Basically, the pages are bookmarked somewhere (though I can’t find them in the library) and the “Bookmark this page (Ctrl+D)” star does not work for them. New and old bookmarks are not affected.
There are about 10 pages in this state, from Wikipedia to news stories.
The problem persists after reinstalling and even without the addon!
UPDATE: The affected bookmarks do not sync to other devices. Other bookmarks do.
UPDATE 2: Removed places.sqlite* and the problem disappeared. It seems that it was corrupted.