I have not had a chance to encounter this problem, as I always use default ringtone for alarms 🙈
But I have checked it on my device now, and it didn't happen to me.
So if it happens again in the future, please be sure to share an error report via Beta Feedback option in Samsung Members app!
It happened again last night after phone restarting 😞
Alarm ringtone was loud and default from the one previously set.
I've sent error report