Aws status page has been updated showing a problem with Amazon Route 53 Resolver (N. Virginia). Heroku status page does show having a availability issue https://status.heroku.com/incidents/2090

Just heard from AWS support. US-east-1a DNS is the only AZ affected. They don't yet have an estimate on time to resolution.

I believe one person's 1a can be someone else's 1b and so on.

Yup, that's indeed the case. Each AWS account sees different names for the actual AZs. That's probably to prevent everybody always using the first AZ on the list (thus overloading the first AZ and underutilizing the remaining ones).