Iphone XR automatically goes into vibrate(no sound/ring/no alarm) mode

My ringer tone volume is set to high and even the ringer mode is switched on. But in the mrng the alarm does not ring with volume. Only vibrates. Besides that, usually we can hear the lock sound when we lock the phone but few times i noticed i dnt hear it in the mrng after wake up. This happened mostly overnight only where the next mrng the alarm doesnt sound and the lock button doesnt has the 'click' sound too. After restart the mobile it works again properly. Why is this happening? How to fix it? (my software is up to date)