For the last book I read and my current book, I've had an issue with location syncing between my 5S and iPad 3, both running iOS 7.0.6. I'm afraid i cannot tell you when the issue actually started happening as I went through a fairly long period without reading and just noticed the issue very recently. I normally read on my iPhone throughout the day, then get into bed at night and read on my wife's iPad. What I've found is that the synced location when loaded on the second device is always behind the synced location that was saved by the first decide. Here is a screenshot from each device with the currently synced location loaded:
As you can see, the iPhone, which I last read on today, is at 13% read, but the iPad, which I've loaded the synced location on, is only at 12%. I have tried deleting all the synced location files from the Marvin folder in Dropbox and forcing new files to be created, but it hasn't addressed the issue (the screenshots above are from after doing that).