Resolved
Our upstream provider has made changes, and the issue with intermittent DNS failures in some regions is now resolved.
Update
Further investigation has shown that default domains (herokuapp.com addresses) are also affected for customers in some regions.
We're communicating with our upstream provider about this issue.
Issue
Customers in some regions are experiencing intermittent DNS resolution failures for herokudns.com addresses. This may result in failures to resolve custom domains for apps in our Common Runtime.