Yeah, with CMC's, and even with good media, a burn that completes is, unfortunately, not a reliable sign that the media it was burnt to is good. Memorex CMC's have, on a good day, a 20% success rate.
However, if you are sure you were burning to CMC's BEFORE with 2.1, you were probably lucky they were okay. It is most likely just coincidence it happened with when you updated. Have you recently changed the brand of media you normally use? Even then, within a package of discs on its own, you can, sometimes, get mixed manufacturer ID's.