My Nook HD goes back to the beginning of the book each time I stop reading it. I don't even exit out of the book to use apps or anything. I am working around it by bookmarking my page, but it is such a hassle to find the page I was on.....especially when it is a lengthy book!!
Is this happening with just one book, just books from a particular publisher, or ALL your books? Have you powered off your device recently?
It is particularly nice for non-fiction. And like the option to change the background to a different color.
For $5, you can get the ad-free version. Spend another $3 for calibre companion, and you have a very nice library management system!