⏰In a nutshell,
If you set one of your music in the library as a ringtone and Alarmy was terminated in the background, alarms will ring with the default ringtone.
Even if you do not force quit the app yourself, sometimes the iOS terminates the app in the background for some reasons.
Please understand that 3rd party apps like Alarmy could be terminated, unlike pre-loaded apps.
For more detailed information, please refer to this article: My alarm didn't ring