Here's the things I can think of that could cause the last read page to change on reopening the book:
- Database corruption can cause the last page to not be saved, but then it will just keep going back to the same page
- kepub with bad span markers would leave the kobo no way to remember the position, and even rotating could make it jump to the closest span marker (renaming a epub to kepub might do this); I've seen bugs in kepub converters do this as well. Only such bug that did this with calibre was fixed long ago.
- Slight mechanical cover misalignment could cause buttons to be pushed. My cover has pockets on the inside of the cover where the buttons are
- (unlikely with a cover) something touching the screen while the device is awake; if the cover is not aligned correctly to keep the device asleep and something slipped under the cover... and if it was on a corner and stayed there a while, you could go into fast page turn mode and end up at the end of the book.