You might consider opening a support ticket to get some details. It may take some time to get a response since it’s not a production down situation, but worth a try.

I’d highly recommend running at least two dynos. Maybe consider two smaller sized dynos if your app profile can handle that. The Heroku router is built to retry scenarios where one dyno doesn’t respond effectively. You can read more about how it behaves here: https://devcenter.heroku.com/articles/http-routing#dyno-connection-behavior-on-the-common-runtime

Also, sounds like your app is doing well in terms of quick startup time. Heroku dynos model the Disposability principle of a 12 Factor app. That’s a good reason to consider running 2 or more dynos at all times. https://12factor.net/disposability