I just updated my device to Android 9 yesterday and downloaded a few custom notification sounds. I set them up and was successful at receiving them as well. But today, a prob occurred and I was not able to hear the notification sound. It was just the flashlight blinking which made me realize that I have new messages. And the problem was for all the apps. Then I turned to one of the system's own notification sounds and it is just working fine. Can anyone tell me why so?
I searched for this problem's solution but couldn't find a clear answer. My suggestion is to start the device in the safe mode and send your phone an sms to make sure if it is caused by a third party app. If the notification works, then you have to clear the cache and data of app's that you think might cause it. Or else, if the problem is still remaining, checking your device at the Samsung Server is advised.