I attempted to take a video to demonstrate the condition of Umbra's ears, but she has never cared to be on the big screen (or any screen). I will try again later, maybe I just need to be stealthier, because she seems to especially enjoy a nice ear scritch when they get all flaky.
I did dig up a Medirabbit.com post I saw last year which seems to be the same as what Umbra experiences. It doesn't give much explanation, other than I'm not the only one who has seen this. I think it is safe to say that it will be a regular occurrence during the summer molt. When I had the vet check for fur mites last year (what I thought the symptoms pointed to in my limited experience), he likened it to something called "slipping" in chinchillas, possibly brought on by hormone imbalance from her recent spay. However, more than a year after spay, I think it is safe to say that it is not the cause, but I am none too worried, although last time I thought Umbra's ears would be bald forever, I now know better.