Earlier quoted context omitted.
Most android devices do that - sleep-state an application and it won't tell you anything until you actively pick up the device and use it. Been like that since I had my Kyocera DuraForce Pro on Android 6 or 7.
Yes, and there is an option to opt-out of that list. I did enable that. I'll phrase my issue differently: Is there any way to have reliable notification delivery that are time critical? Is actually calling the only 100% reliable option, as the PagerDuty does?
Note that you can only have one push notification reliably per user interaction. So once you have sent a notification, further ones won't be reliably delivered until the user interacts with your app in some way.