I have set my alarm a couple of times, with just the vibrate option set - so that it would wake me without making lots of noise. Unfortunately I thought that I slept through the alarm the first time, and then wondered when it happened on the next occasion. I checked the alarm and sure enough it did a good job at vibrating at the correct time, during the day.
Then it struck me - I have my automatic 'Do not Disturb' times set from 11 pm until 8:00 am - I really do not want emails to alert me during the night. But it seems that 'Do not Disturb' includes silencing alarms - which is pretty silly.
I don't think I am doing anything incorrect, and I don't think that I have missed a setting - but if I have, I would love to know.
Otherwise, I would love Garmin to fix this quickly. My Vivosmart HR+ did the right thing and silenced email alerts but sounded alarms during 'Do not Disturb' hours.