Do these Alerts diminish when you are directly handling the iPhone or does it happen even when the iPhone is locked and you are not looking at it?
——
If the issue is happening while you are handling the iPhone, then This is part of the Attention Aware Feature of the iPhone. When the iPhone has your attention while you are looking at the screen (typing, reading websites or working with an app), then it automatically reduces the sound because it knows you are looking right at it so no need to issue loud notifications.
You can turn this feature off if you want by opening Settings ➔ Accessibility ➔ Face ID & Attention ➔ Attention Aware Feature set this to OFF.
Axel F.