turning the page and having the corner bookmark and the book close is what the book does when you finish it.
i've had a similar problem where it did this after flipping from the first page...i believe there was an error in the book and it believed the book was only one page long. sounds to me like it is the same thing happening...so the bookmarking is not relevant....it is just that it either can't tell that the book is longer, or the whole book wasn't loaded onto your reader.
i can't remember how i solved it other than putting books on and off and possibly signing in and out of my kobo account and then reloading all my books. it happened when i was still figuring out my reader and hadn't done a lot of actual reading on it. but if taking on and off doesn't work, i would definitely recommend signing in and out of your account. though that does take a bit of time to get books back on.
edit: oops...GeoffR already gave the same recommendation...so ... +1 to that