iPhone volume lowers when a text message or email notification is being displayed on the screen

That is a featured called Attention Aware. If it knows you are looking at the screen it will quiet sounds because it knows you are aware of them. If you don’t like it go to Settings/Accessibility/Face ID & Attention and turn it off.

jengel59 wrote:

I have an iPhone XS MAX running the latest IOS14.6.

Anytime I am on the phone and a text message or email notification is displayed on the top of the screen the phone volume level lowers and then returns to normal once the popups are gone or I swipe upwards to get rid of them faster.

I don't recall how far back it was when I didn't see this issue, maybe ios13.

A quick fix is to slide the side lever on the iPhone down putting it into silence mode while talking to someone. Not a fun thing to remember or due.

Anyone have a fix for this or can point me in the right direction?

That is how it work, when you receive a notification you look at your screen, assuming you have face id enable, it recognizes that you've seen the notification, so it no longer needs to ring at a high level.