Iphone alarm sometimes doesn't ring - iphone 7 ios 12.0.1

Try updating to iOS 12.1 (which is the latest) and restart your phone. * you mention you are on 12.0.1 which is previous)

While your ringer being on or off doesn't impact your alarm, ringer volume settings will affect it. If kept on the lowest ringer setting, the alarm may not be loud enough.

"Alarms match the volume you set for your ringer," Apple notes on its support website. "If your alarm volume is too low or too loud, press the volume button up or down to adjust it.

You can also go to Settings > Sounds to adjust the volume."

Also, try setting an alarm to go off in minutes. Watch the phone and see if it does the same thing.

How to set and manage alarms on your iPhone - Apple Support

That won’t work either. The alarm icon won’t even appear on the top right. Yet I go to the alarm app and it’s on. Still doesn’t go off

Also, check this...if everything else doesn’t help.

https://support.apple.com/en-us/HT207512

• Set the volume on your iPhone. If your alarm volume is too low or too loud, press the volume button up or down to adjust it. You can also go to Settings > Sounds & Haptics and drag the slider under Ringers And Alerts.
• If your alarm only vibrates, make sure that your alarm sound isn't set to None. Open the Clock app, tap the Alarm tab, then tap Edit. Tap the alarm, then tap Sound and choose a Sound.

It’s probably because either the volune was turned down or off.

go to settings>Sounds> ringers and alerts.

the button should be white.

This way the volume of the ringer and alerts will not be affected by the volume button.

Also, make certain that your alarm alert is set to a sound and to NONE.

12.1 hs been released.