Does this happen with every book or just one?
If every book, I will download the latest KindlePreviewer/Kindlegen and build myself a testcase.
If just one, I would be happy to look into this but need to have a testcase. So if that is the case, please post a testcase here or privately mail KevinH with a link.
I will try to figure out how to modify the splitting code to make it work (or workaround that issue).