I think some of my bookmarks vanished?

I am confident that I put bookmarks on stories that I've read whether it's complete or on going. However, I checked some stories which I definitely read but there's no bookmarks at all. Is it just me? 'Cause I want to continue reading but I don't know where to start.

I always make sure to bookmark the stories I read so it's definitely not because I forgot. It also happened a lot already.