I see dynos for apps in a Private Space restarted around the same time. What triggered the restarts?
Heroku upgrades the control software of Private Spaces regularly (typically every couple of weeks) to fix big bugs and add new features. As part of the upgrade, the platform restarts dynos with rolling deploys onto newly provisioned runtime instances.
For an app with one web dyno, this may lead to an extended duration of being unavailable. For an increased availability, we would like to recommend running two or more web dynos.