Hi there, I am sorry about this inconvenience.
Your domain mapping seems to be working just fine at the moment, and I am not able to reproduce the issue that you had. I am sorry resetting the primary domain was not helpful.
Did you update the CNAME record after reporting this issue, by any chance?
Once a CNAME record is set correctly and domain mapping active, it works fine. If you cancel and refund the domain mapping upgrade (you can do it under Store > My Upgrades within 30 days of purchase, if you wish), your subdomain will no longer be mapped to your WordPress.com blog.
Please let me know if you run into this issue again and I'll be happy to do a further investigation.