I have not found the problem to be specific to particular epubs. Every epub I've looked at shows the problem. Some of the italics (also bold but there are less occurrences of these) in the book display correctly and some, in the same book, don't. If I close the book then reopen it the italics which were correct originally, may not be anymore and vice versa. Unless you are checking the same epub, page by page, on 2 different readers (which I did) it won't necessarily be obvious when italics are missing. Of course some might say, if it's not obvious, why on earth does it matter?
@log, I tried replacing font-weight:bolder with font-weight:bold, but it made no difference in my small test epub.
If anyone does come up with a definitive set of css rules to ensure bold and italics stay put in any epub, I'd love to hear about it.