Change effective on 17 September 2018
Currently, when adding a custom domain on Common Runtime, Heroku currently specifies a DNS target based on your domain name e.g.
my-domain-name.herokudns.com. On October 16, 2018, Heroku will change to a randomly generated DNS target scheme, e.g. whispering-willow-5678.herokudns.com.
You should always use the DNS target provided by Heroku Dashboard or API for your DNS configuration. The change only applies to apps and domains on the Heroku Common Runtime and existing custom domains will not be affected.