Why don't my domain's messages to a google group get their headers rewritten so DMARC can pass?

I did finally get this response from Google. I was glad to find someone who understood the issue, but unfortunately it looks like Google is sticking with their incorrect treatment of the DMARC directive.

Google Groups will only rewrite the From: header when the DMARC policy of the original sender (you in this case) is set to reject or quarantine.

With DMARC/p=none, DKIM isn't rewritten when sending to a group, but because DMARC is set to "none", it doesn't matter that the group will break DKIM from the point of view of delivery and spam detection - it will still get delivered properly. Unfortunately, it does make it look confusing if you're monitoring your deployment.

In order to see the correct behavior you can switch to p=quarantine and monitor the delivery. Please tell me if there is anything else I can assist you with, I'll be glad to follow up.