Yes, I'm having problems as well with missing text in the review questions as well as some other formatting issues with floating text boxes and text-wrapping.
My book is in English with Japanese. I'm running on a MacBook Pro (Mid-2010) with OS 10.9.1 and iBooks Author 2.1.1. I have changed the fonts many different times. I've tried adding extra spaces as someone else mentioned elsewhere. I've tried every conceivable mix of layers. I've tried with a fresh new file. All to no avail.
I have put a lot of time and effort into my interactive Textbook and have finally finished. Now these issues are the last remaining before delivery through iTunes Producer.
FYI, I have previewed the book in iBooks (latest version), on an iPad mini retina, on an iPad (4th ten); both as previews and using the exported .ibook format (all on iOS7). The problems persist.