My X would also ring loud then go low.

Yes, having Attention Aware turned on will lower the volume of notifications if you are looking at the screen. It’s a very handy feature on the iPhone X.

If it was loud and then went low you probably were looking away from the screen and once you heard it and looked at your iPhone it immediately lowered the volume. But yes having Attention Aware disabled will not lower the volume when you’re looking at the screen and also won’t require you to look at the screen to unlock with Face ID (it will just scan your face without you having to look at it).