Got a paperwhite. When I read a book, and just one book, the kindle correctly tracks my place in the book. Closing and opening the cover works just fine.
But if I go to the menu and choose another book, then back to menu, the reading location on the first book has been cleared. If I open it, I am started back at the beginning...losing my place entirely.
I have searched, but can't seem to find anything referencing such behavior. I'm using sideloaded mobi files.
For illustration, a YouTube video I made:
http://www.youtube.com/watch?v=zyAiIdrvjAE