Comment by zzh8829

Looks like Heroku uses NS1 as the upstream DNS provider. you can find the information like this

  dig NS @1.1.1.1 test.herokuapp.com -> fail

  dig @1.1.1.1 test.herokuapp.com -> fail

  dig NS @dns1.p03.nsone.net test.herokuapp.com -> works

  dig @dns1.p03.nsone.net test.herokuapp.com -> works

So my conclusion is that NS1 is having issue responding DNS queries from other DNS servers. Interestingly, there is no public information on heroku being dependent on NS1 or any current outages from NS1 status page.