In Android, Notification categories, when created, have specified attributes such things as sound or silent, and whether to vibrate or not. Once the category is created, the app itself cannot change those attributes, only the user can. And any new notification that is in that category will assume that categories attributes.
This was done by Google to prevent nuisance or spam notifications from applications from bothering the user, as the user can always adjust the notification category to make it silent or prevent the notification at all.