If Notifications are not behaving as expected for a particular notification type, disable (i.e., turn off) the Notification that is not working as expected - then exit settings entirely.
Next, force-restart your iPad:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
After your iPad restarts, revisit iPad Settings - and again set the Notification settings as you would lime them to operate.
The process of disabling and re-enabling the required Notification type, coupled with the restart, will hopefully clear the problem.
Notifications and associated settings have been heavily revised in iOS/iPadOS 15.x. While they work broadly as before, the settings are now significantly less granular. When compared with iOS/iPadOS14 and earlier, some functionality has been lost - and the new settings structure is counter intuitive; settings can unexpectedly disable notification sounds.