I don't have the app because it's worthless to me. There are reviews all over the internet over the past two days talking about this very thing. I posted a link above that has comments from people who have experienced this.
Set an alarm for two hours from now. Close the app and exit from it by removing it from the task bar. Open another app and let it sit for those two hours and turn on do not disturb as well. See what happens.
Why would you remove it from the task bar. That's just stupid. How do you expect any app other than core apps to do anything if you shut them down completely. You are going out of your way to **** it up by closing it? It's iOS not android, you don't need to micromanage every app to keep your battery from running out.
Set the alarm, press home, carry on using apps. Press the power button to turn the screen off, at specified alarm time the alarm sounds and the notification displays on screen, swipe to snooze. Not difficult at all to use now is it????