No message notification sound alert on samsung note 20 ultra, I just noticed after fiddling with the notification setting on messaging app...
Where it says "New message check", no matter how many times you turn it on, it turns off automatically. I think that's the problem. I think it turns itself off when you get a new text.
Are you using the stock messages app on your Note 20 Ultra?
Are you using any themes or third party launchers?
I would recommend testing your device in safe mode. If your phone is misbehaving, it's possible that a third-party app is causing the issue. To figure out what app is acting funny, you can use the Safe Mode feature. In Safe Mode, your phone won't run any third-party apps. This allows you to easily identity and remove the app that may be causing the problem. Give this link a try: https://www.samsung.com/us/support/answer/ANS00062983/
Next step would be wiping the cache partition on your device. The system cache partition stores temporary system data. It's supposed to allow the system to access apps more quickly and efficiently, but sometimes things get cluttered and outdated, so a periodic cache clearing can help make the system run more smoothly. Give these steps a try: