Alarm unreliable

I have used this app for years without issue, however recently the alarm has failed to sound on several occasions. As others have noticed, the display is acting like the alarm is sounding, however there is no audio alert. I do have Background alarms enabled, the phone is not muted and the phone volume is set at max. I dont know if a recent iOS upgrade caused the issue or if was the last app upgrade, but currently an unreliable alarm is of no use.

AllenEagleFan about Alarm Clock Pro, v1.13.1