I am currently running Android Pie with the latest security updates on a Pixel 2.
On my phone, with PagerDuty 5.27 if I put my phone in to Do Not Disturb mode, notifications still come through at absolute minimum volume and vibrate until I block notifications from being displayed in the application notifcation settings.
This was similar to a bug in earlier versions of Pagerduty on Oreo 8.1 with PagerDuty 5.22.1 where notifications will still alert when set to anything other than total silence.