I have confirmed that this is reliably repeatable in all cases. Notifications will ring, put a toast alert in the shade/lockscreen every time, but only if the user does not have the PowerApp open/up on their iPhone. I can't tell if this is by design or if it is a problem. Because the phone screen can stay on while they are moving about, I really wanted the alerts to make noise and come through regardless of whether they are looking at the app or not.
Just to clarify, this is what I am experiencing with iPhone 7:
Screen on, my PowerApp maximized/in focus = NO alert
Screen on, PowerApps organization screen maximized/in focus = NO alert
Screen on, Safari maximized/in focus with PowerApps running in background = Alert
Screen on, Safari maximized/in focus without PowerApps running at all (but still logged in) = Alert
In both cases where the alerts do not come, turning the screen off using the button on the side of the phone causes them to come.
I have tried changing every setting in both the general alert settings and the PowerApps alert settings on the iPhones and none changed this behavior. Text messages and other alerts behave as expected without regard for the screen being on or off or what app is in focus.
Can anyone help me understand what is going on here?